It always seems to take me a fair bit of looking to find the image I want for any particular 'shop.
I tend to download a lot more images than I ever end up using for a particular project. I like the freedom of changing my mind at the last minute, and while I'm looking for a good source pic I inevitably find a lot of others I imagine I might use for future projects.
So what I do is [I'm on Windows7 but I know at least part of this is possible on other PC OS' and Mac]:
- Copy the url of image [i.e. the link you would use in your 'source' posts]
- Right-click on the image file once downloaded, go to Properties, Details
- Under Comments*, past the source URL
It sounds like a few steps to do but once you've done it a few times it's a really quick method. Especially considering the alternatives, and especially if you keep your download destination window open.
Hope this can make some people's lives a bit easier! And if anyone has an even better method I won't be insulted if you enlighten me

*Or one of the other options such as Title, Subject, Tags, Authors, Copyright...if you use Bridge you might find using one or more of these fields more helpful.