I'm not sure since I've only been doing tiles here for about a month now, but I get the vibe from the other folks that straightup photo manipulations aren't really good for the quilt.
I find them very distracting visually since they are photo-realistic compared to the other tiles on the quilt (well of course they are... they are PHOTOS).
Outside of that reasoning, I just don't like them personally because I find photo-manipulation less challenging than the more traditional freehand sketching/illustrating/painting that you find on most of the tiles here. I have hundreds of interesting photographs that I might weave into a tile and repeat a few patterns and textures from surrounding tiles, but that would just be too easy. :\