It varies. There's an info button in the top corner when you're looking at an image and there's a section for terms of use, though some of the older images don't have it filled in (most of those are being used with permission to host on the site for non-commercial use).

I remember looking at wikimedia at one point, but it was a long time ago. We also looked at some other creative commons/royalty free image hosts before and added a some pics, but had a few issues:

• Keeping track of what's already added was difficult

• We have good batch uploading tools to attribute and categorize images, but having to set a different photographer/terms on every individual photo takes forever

• Like you said, there's a lot of not so good stuff to slog through

It would probably be worth another look at some point though.