For my reference, I've written up my vague notions on the meaning of copyright. You can read them here
Take for example Disney. If you draw a mouse who looks mostly-sorta-kinda like Mickey and use the image to decorate something completely unrelated, such as that book, "Tuning the Engine on your F16", that you've been writing, you'd be infringing on Disney's copyright. This is because the image of Mickey (even though you drew him stroke for stroke by yourself) is recognizeable as Mickey and as such, could be argued as contributing to the sales of your book. The exception is if you are making a parody, but even in this case, I think it depends a lot on how much each party can pay their lawyers.
Okay, I'm a little fuzzy on details, so if any lawyerly types happen by and want to set me straight, go ahead.
Let's watch now and see if Disney gets annoyed that I used Mickey as an example... :-)