Unit Testing and External Resources

[ownedboy]

I'm kind of new to unit testing, but I've recently seen how it can be quite useful. I've seen that most unit tests are self running. In fact most unit testing frameworks provide a way of running several tests at once (such as unit testing a whole system).I wonder though; How do you deal with external resources in self running unit tests? I like the idea of testing a whole system and seeing which classes failed, but a class may, for example, create thumbnails from an uploaded image. How would that test be self running when it relies on an upload image? Would I keep a directory of images, and "pretend" to upload one of them in the test?Any thoughts on the matter would be greatly appreciated.