The majority of them come from http://wallbase.cc/ - Which is just a website that compiles images gathered across the web for users to browse and use as wallpapers. I can credit it, sure, but the reason I put my personal website is that I host a similar gallery. However, I understand that is riding content that didn’t originate with me for traffic, but it didn’t originate there either. This is commonly referred to as diffusion, and almost the entirety of the world wide web is built on that. A whirl pool of media.

I mean, like I said, if I link back to the original source it’s no different than being native to tumblr, just another cesspool. If I know the author, I have no reason not to credit them.