This is one of those 'why didn't I think of it' resources. Pinterest essentially lets you bookmark and share images that you like. You can create boards or sets of images, annotate them and share them with friends. It looks something like this:
Simply add the 'Pin It' boomark to your browser and then when you come across an image that you like, it's a click to add it to a collection. Now of course, you can do this using traditional bookmarking schemes, but this makes the process much easier and it's much more enjoyable - and easy to see the images that you've got.
I can see a wide range of uses for this - Pinterest suggest boards for books you like, home style and so on. I can think of subject boards, motivational posters, information based images, library related cartoons and so on. I've had an account for less than a day and it's already really busy with people following me back, sharing images and so on. It doesn't take much work, and it's very simple. Worth while trying it out.