hingyfan,
If you have GraphicConverter, the picture currently being viewed can become your desktop picture. (At least in v5.9)
Simply select the Picture drop-down menu and scroll down to
"Use Picture as Desktop Picture" — Bingo!
If you don't have GC, why not? It's by far the best value in graphics software at $30.
Indispensible for photographers and people who do graphics on their Macs. Absolutely phenomenal shareware which means you can try it out before purchasing.
My 2 cents worth.
Jack