The guidelines are simple.
Please be sure to submit your work to the appropriate folder. Stock Images is for stock images, Textures is for textures and so on. If you accidentally submit an image to the wrong folder, it will be moved to the appropriate one as soon as possible.
Guidelines for Stock Images:
*Ganked directly from this news article by LawrenceDeDark.
Size must be a MINIMUM of 600x600 pixels (excluding vintage portraits that are relatively small and of low quality)
No heavy distortion due to blur/noise/grain (this rule is case-by-case depending on the usability of the image in question.)
No webcams or cell phone images
No sexually explicit imagery
MySpaceFace-style "portraits" will not be accepted*.
*Not ganked from news article.