PPC on the ones that are just starting out and problably CPM networks on the larger ones. When you upload a file, you gotta give the link out.. so when someone clicks the link to view the image that you uploaded, they will probably be presented with an ad somewhere on the page alongside your image.

I'm guessing that's how most of them make money today. When I had my image uploading site, I depended on AdSense to bring in money. Not much though, probably about $0.50 a day.

Edit: Ooh, I just noticed "file" uploading site. That will include the likes of YSI, filefront, etc. too right?