I don't think a cap really has an effect on the time a picture stays on the front page. Do you think the time needed for one user to post six pictures in a row and then another user to post one, is significantly different than the time needed for the first user to post three pictures, wait for the other user to post one, and then post the other three ? I don't believe so.

Your pictures will be pushed away from the front page, small cap or large cap or no cap. Personally, I visit this site daily, and there's practically always more than one page of thumbnails to visit. I just browse backwards until I encounter a photograph I've already seen on a previous visit. I would hope everybody does that, because after all, seeing new pictures is what people visit this place for.

What a cap does accomplish, is forcing people to make a selection. So the average quality or variation should be better with a cap than without. But the cap shouldn't be too small. Surely I'm not the only one who likes series of pictures of an evolving scene or of a common theme. A cap of 2 is simply too small for such an endeavour. 3 or 4 seems a lot more natural to me. But I have no problem with 5 or 6 either.

So my vote goes to: leave as is. If someone breaks the cap, simply send him a friendly update on the site policy, and possibly remove some pictures if it was really excessive. Most people should be reasonable enough for that to be sufficient.