You are out of your ever-loving mind. If I buy, say, a lithograph you sell of one of your original works, I scan that and use it as a computer desktop background on my personal laptop, I am not stealing from you. I bought it, if I want to scan it and put it on my personal computer as a background, I certainly can.
You *sold* it to me. I *own* that lithograph. I can repurpose it for *personal* use all I want. (Like right now I'm thinking about using it as toilet paper.) It's called fair use. Fair use! FAIR USE!!!! FAIR USE!!!! FAIR USE!!!!
You can indeed use the lithograph itself for anything you like. However, what you can't do is COPY IT! (Not legally, anyway).
I've done the same myself (copied pictures for wallpaper) butI know it's illegal and I also know that no-one's going to bother pursuing me for the crime; but that doesn't change the fact that you're not really allowed to do it.
Steve