Fitting a desktop background into my desktop screen

The best (and probably most expensive) way would be using Photoshop.

If you don't own that app, try ImageWell (freeware):
http://www.macupdate.com/info.php/id/11375

Or GraphicConverter - very usefull (but shareware):
http://www.macupdate.com/info.php/id/263

Yes. Do not uncheck the box at the bottom that asks for recalculating. ("bicubic" is okay for that purpose.) You might have to crop the picture after that to get the best part of it. Or you have to add some solid color as a background. Both can be done by using the canvas menu item.

To avoid color edges in the gradients you can use the "add noise" filter with 2% or 3%.

When saving it, use JPG with the best quality. Save it to the pictures folder in your home directory and you can choose it easily.

P.S.: The image dimensions you've given represent the worst case! It would be much easier to rotate the screen device!! 😉

GIMPshop is a very good alternative to Photoshop if you don't want to spend the bucks to buy Photoshop:

http://plasticbugs.com/?p=341

alright thanks. how about those that have dimensions
SMALLER than my original desktop screen size which is
1680 X 1050. (maximum for my 20" iMac)

Well, that is a question of quality of the picture. The more you scale, the more blury the desktop picture gets. It is a matter of fact, that you can't "press" more details in a pic than it has in the original version.

Try experimenting with those setting like "bicubic" when scaling >100% . You get best result when scaing 200, 400, 800%.

BTW: All these tipps are not photoshop specific!