Of course only delete the jpeg's - not your original layered files. That way you can still make changes in the future.
Also, I have heard (this was about a year a half ago) about photobucket doing clean sweeps of their site. What I mean by that is that they went through their site and deleted Disney (or copyrighted) images. It was when I was first active on the boards but not yet designing, but I remember all the designers talking about how all their images had been deleted by photobucket.
But if you are talking about designs you have done for others that you no longer need - I would say yes, delete them. I know what you mean, I had to buy a bigger (if that's the right word) computer to hold all the Disney stuff I was saving.