I tried to sell some images in a small scale and was surprised when it turned out well. Only thing is that some of the potential buyers wanted larger images. I´ve only rendered them in the format 800x600 with 256 rays per pixel. Can I enlarge the images without getting heavy pixelationproblems, or do I have to re-render them? In which case I have to let a printing-company take care of them when it comes to printing. I´m very novice at this kind of things so any advice is greatly appreciated.

render to disc at whatever size required at 300 dpi to get the best quality.
rule of thumb..

1. if you lower the dpi..you can increase your image size..oh photoshop, that is.
2. you can increase dpi if only you reduce the image in size.
3. you can Never ever increase size and not reduce dpi.

put it this way..stoner..imagine pixels as being fluid in a little confined space.

you squash the space.. ie..make the image smaller.. keep the same amount of fluid..ie..dpi..and well.. the color stays good.
imagine widening up that space..asking the fluid to spread itself out.. of course..it's gonna look really spread out, very little honey over a slice of bread. can't cover the whole area etc..ie..pixelated.

and the rest..you can figure it out...

have fun...

Thank you erosiaart

A follow-up-question. How about the render-times when rendering to disc? And how importantant is the number of rays per pixel?

more rays..more time.. you have refraction/ reflexion/ glass mats/ everything affects the render time. i still haven't 't gotten bryce 6 as yet..will do that next month..but if going by 5..you can't stop rendering to disc..you have to let it go on rendering..takes up 100% of your cpu process. i believe  on bryce 6..that's a bit better..as well as you can stop render to disc and save. correct me..all those who using bryce 6 now.

http://www.renderosity.com/mod/forumpro/showthread.php?message_id=2567570

http://www.renderosity.com/mod/forumpro/showthread.php?message_id=2707297

try these  posts - fun to read.. your render times can be anything..i've had a 3 ft x 2 ft render take me 3 weeks because i had lots of glass mats, and that too..refracted in water. or it can take a day.

I always think of DPI per inch.  If I set myself at Magazine quality at 150 dpi,  a 10 inch image would have to be a render 1500 dpi in size.  You could double the size, and get approximately screen resolution (72dpi) or halve the size to get art quality resolution (300dpi).  Everything I know is in agreement with Erosiaart. 

I will say I did this once, re-rendering my picture "September Freight" to make an 11 inch wide image.  That's a long render when you only have 512meg of RAM.  Even at standard quality.  And the file is HUGE.  I had a 21 inch version of "Autumn Station" that was one gig unto itself!  Needless to say, when the buy backed out, I deleted it because it was taking up too much room.

It all depends on what you need.

mdb

just resample it to a larger number in pixels and redefine the dpi as needed or keep it aqs is, or don't even worry about it - some intelligent software like freeware Irfanview to on-the-fly dpi matching when you print. It's all about the pixels you have and supply to the printing process anyway, whether you tell it upfront how many dots to cram into each inch (dpi) or whether it resamples it intyernally makes no difference.

However, you might do this trick: instead of one big resampling from 800 to 3300 pixels (e.g. to match 11 inches at 300 dpi) you might see better results buy resampling not in one fell swoop, but instead in several steps, perhaps going up in increments of 10% to 20% or so, That depends a bit on which algorithm is used when resampling, bell, spline, triangle, lanzcos etc... but in general this can help avoid 'pixelation' or rather a blocky appearance of the few (800) pixels being resized to about 4x their size.The resampling helps in making sure that new pixels are produced in-between by interpolation, but a big jump from 800 to 3300 pixels is a big step and might still leave some artifacts. Doing a few small step resamples to eventually reach 3300 pixels may avoid some of that. Irfanview is a must have for this, so much faster than Photoshop.

I use Irfanview anytime I print something because of the last-minute decisions to print a slightly different size than original image dimensions and dpi might have it.

DPI and PPI are very different things. PPI is what matters in the file. It is the number of pixels that make up each inch, thereby defining how detailed and fine the image is. For small prints (8x10 or smaller), you should have close to 300 pixels per inch, or approximately 2400x3000 pixels for a "flawless" 8x10 print. You can get by with 200 ppi without many problems, but you will be able to notice a lack of fine deatils if you go right up close to it. As the print gets larger, the PPI gets more forgiving. For example, once you hit 11x14 inches you can get away with 150 pixels per inch, because artwork at this size tends to be framed and people do not stand as close to the image. So for 11x14 you can render at 1650x2100 pixels. If you look up close at it you will definitely be able to see the small pixels, but if it's framed you won't see this. You can stay at around 150 PPI up until about 20x30 inches, and then you can drop it down to about 100 ppi. I would not go below 100 ppi unless you are printing a very large image (4x6 feet or larger) that will only be viewed from eight feet away or more, as people do tend to stand close enough to see the pixels otherwise. DPI, or dots per inch, is a function of the printer you ar using. It is the amount of dots of ink for each inch on the paper, and this number is always much higher than the number of pixels in the image. When you think about it, each dot from the printer is round and "fuzzy" as it is absorbed into the paper. If there was only one dot for each pixel, the image would not be very detailed at all because you are trying to define a sharp, square pixel with one fuzzy, round dot. As such, you usually need to print at least at 720 DPI which will produce about four dots of ink for each pixel (assuming the original image is at 300 pixels per inch). This will give each pixel a rounded square appearance. For detailed photos you need a printer that can handle 1440 DPI or better. This ensures that at least 16 dots of ink are being used for each pixel that is printed, esuring that the pixel looks very close to a square and will not blend into the other pixels around it due to dots are too large. For museum quality prints I print at 2880 DPI, which uses something like 64 dots for each pixel. Between 1440 and 2880 there is very little difference to the untrained eye, but you can see it in photographs where there is a lot of very fine grain such as in black and white scans. Also, I can tell you that most printing places will overcharge you terribly for prints and you're better off buying a decent printer and doing them yourself. Get an epson R1800 or R2400 printer and you can make prints that are better than anything you can buy for a third of the cost, and you can get them right away! You also have the benefit of getting to choose your papers and have much more control over the final output. Just my 2 cents In closing, you should re-render at about 2000x3000 pixels, and this will allow you to make prints up to 11x14 with no issues.

Attached Link: Printing Article

So very helpful of you all. I wish I had the time to be of so much help to someone else. I now have a lot of info to digest, and that may take some time.

I just sent some work off to a commercial printer. 300dpi was the ONLY acceptable setting.