Appropriating Images

I was reading some blogs again, and came across Caitlin's about appropriating images. I couldn't agree more with her. I guess I never understood why people do appropriate images. If I were to do something like that, I think that I would feel like I was cheating. It just has never seemed right to me. I had never even thought to do anything of the sort, until I came to UB, where I feel like sometimes it is almost encouraged. I feel guilty if I piggy-back an idea off of another artist. Like Caitlin said though, where do you draw the line? I guess I have just never understood the logic behind it.