when i render an image and export it it says compression 75 i dont know what it means..but if i render an image at 2,000 X resolution it is all pixelated..i tried changing it from 75 to 100 with no difference

That is JPEG compression. Depending on what version of Poser your using, P7 I think you set to 100% and the other versions were the other way.

It is better to save the render out in one of the lossless/ alpha channel formats such as Tiff, which I prefer or PNG or PSD? does poser export PDS? ca't remember. But this way you can import the image into your favorite paint program and do all your postwork then save out at what format and compression you want.

Poser exports PSD. I always use PSD and resave from Photoshop.
PNG and TIFF are also good options.

you need to change your render settings under render dimensions, make it 300dpi.  after you render it, export the image, that retains the 300dpi and save it as they said above, either a tiff or a psd layer.

The dpi settings work in conjunction with the image size in inches. A 8"x10" image at 300 dpi is (8300) x (10300) = 2400 x 3000 pixels.
Doesn't matter if you set the image to 2400x3000 pixels, 8"x10" at 300 dpi or 32"x40" at 75 dpi, it'll result in exactly the same amount of pixels at exactly the same quality.
Save in any lossless format, and you'll keep the quality.