As said in previous chapter, the hottest sellers at the moment are vector illustrations. This is completely understandable because it takes certain technical skills and talent to make them (therefore they aren't easy to find) while end-user finds them more usable than bitmap picture.
If you are skilled vector illustrator then all you need to know is that you should prepare your works in EPS file format (any vector-based software such as Corel Draw, Illustrator or Freehand will happily export your images to EPS) and that you need to prepare low-res bitmap previews (450 pixels height/width is enough) that you'll submit along with the vectors.
3D and digital artwork artists should export to high resolution bitmap JPGs, just like if the images were photographs. As for the photos, the following is great: you are allowed to Photoshop them. You may remove background around the objects (designers often need that!), you may adjust brightness, hue and saturation, you may combine photographs and you may even run Photoshop effects - all of this as long as photographs are 100% yours.
But is this all worth your time? Will you make enough money to make it all worthwhile! Click here for profit estimates!