MacOS X instructions
Step 1: Select the correct size of the wallpaper.
It should be exactly as big as your monitor screen (which is a little bit bigger than your browser window).
Step 2: Right click on the picture to bring up a menu list.
Select "Save Image" or the menu item that has the same meaning.
It's important that you remember where the picture was saved.
If you didn't get to choose where to save the picture, it was probably saved in your default download folder.
Step 3: Open "System Preferences" and choose "Desktop & Screen Saver".
Then select "Choose Folder" in the listbox to the left and locate the picture that you saved in step 2.