Those are the rules on gtplanet.com, and i think they are pretty good for us too..?
NOT ALLOWED:
Alter the geometry of the car. No stickers, or body modifications not available in game.
Add shapes or objects from outside of GT5, signatures excepted.
Remove items from shot (scenery, other cars).
Use more than 1 image (HDR, photo-stitching, multi-exposure).
Add effects that make the picture look something other than a phorograph.
ALLOWED:
Crop and/or resize an image.
Adjust the level or curve values.
Alter the colour tone of the image (gradient maps, photo filters, colour balance, etc).
Adjust brightness, contrast, exposure, shadow/highlights.
Enhance shadows or highlights (dodge and burn tools in Photoshop, for example).
Add noise and/or diffuse glow.
Add a post-game tilt-shift blur, or apply a motion blur to a stationary Photo Travel shot.
Make use of Layer Blend Modes.
Use a Sharpening filter and/or tool.
Scale or enlarge rims.
Remove or modify license plates.
Remove the GT watermark.
Add a personal signature.
Clean up or fix jaggies, texture pixelation, or colour separation.
Add a vignette effect.