Changing Desktop wallpaper

How do you change the desktop wallpaper to a picture or image that you see on the web. In windows I just right click and choose set as background. What is the equivalent in Mac os X.

In general, you need to save off the image in question to your hard drive. Then you can use System Preferences, Desktop, Desktop tab, to set the background to whatever you want. Alternately, many good Mac image programs will do the same thing for you - they provide a "set as wallpaper" option for the image you are viewing.

I've got a folder under "Pictures" called "Desktop Pictures" -- if I want to use a picture I find on the Internet on my desktop, I'll simply drag it from my browser into that folder. I've got my preferences set to automatically rotate my background every 15 minutes, so any new images I drop into that folder will appear automatically.

If you don't want to rotate, then you can select it manually from system preferences.

ok cool guys, I really appreciate the help. I have been a switcher for about three weeks now and I am loving mac os x tiger. I chose to go with tiger instead of buying a new vista machine. I just go fed up with windows even though I still have to use them at work and my wife and I still use a windows machine for the "family" system. Slowly but surely I am taking my stuff off of the windows machine and transferring it to a external hard drive so that I can access all the itunes, and files from both computers. Currently I am using an old ibook clamshell 466 to learn mac so my experience is a little tainted by such a slow machine. However I just purchased a powerbook 1.33ghz G4 from ebay. So hopefully this should improve things.

Firefox lets you do it with a right click on any image:

Pictures have to be at least a certain size to fill the screen without looking grainy, though. Am I right?
Something like this size: 1024 x 768 pixels

Yep, closer to your actual resolution will yield better results.

I found another way to apply an image as my Desktop Picture on my Mac using OpenMenuX, which I bought after trialling it.

Thats only healthy bresktop (breakfast desktop). Corn flakes. Fruit. Jammed toast.