People upload images from where ever. Some take pictures and scan. When it comes to covers and promotional images, they're typically given out to retailers and the press for free anyways.
It's not like photography were the image
is the product. By showing a photo, unlicensed, on a site you're basically bootlegging the product. Cover art on the other hand is not the product. It's a representation of the product. While the imagery is technically copyrighted you typically don't see game publishers going around knocking down sites promoting their products. (Unless its YouTube Let's Plays, LOL.) It's not like VGCollect is selling posters or shirts of the cover art.
This site is pretty much non-profit anyways. (There's even a donation system.
) Other than the admins who moderate the entries, there's really one guy running the site. And he's running it on his spare time. I think the worst thing that could happen is to be told to remove the images.
This would be a great question to bring up to Wikipedia.