For photoshop, you can start by posting over in the photoshop forum here on renderosity.  they'd probably have more of the 411 your looking for on techniques.    Basically, you have to wade through a lot of tutorials and try them, its not an instant process in learning.

I used to use Paint shop pro, and found photoshop too daunting..... what a mistake.  When i finally bit the bullet, and dove in, wow, what an improvement!  Not saying PSP is bad, just you can do more with photoshop.

The best format, unless you want a transparent background is a PSD file.  BMP's are TOO huge, and are not very reader friendly.  Try uploading them somewhere and you'll see what i mean.  PSD's keep the quality good.

Bigger is not always better, if the image is HUGE, you'll find this lag in working, where you try to paint something and it takes four minutes or so for the paint to actually show up.  The trick is making it big enough, but also the higher dpi.  render your images at at least 300 dpi, then you can expand and contract it without any real loss of content structure.