A little time-wasting activity of mine currently, is customizing my gmail theme. But, as I look around, I cannot find the option for adding a background image. Some themes have it, so I guess, it is possible.
Where is it ?
You can't - if you are going to create your own custom themes.
Background pictures are currently only supported for themes that are either 1) by Google themselves, or 2) by authorized artists' themes submitted to Google.
If you are really dying to tweak the LooknFeel of GMail, you might want to give Firefox + GreaseMonkey a shot.
here's another FF addon "to tweak the LooknFeel of GMail":
Google Redesigned - A complete redesign of popular Google services, Google Redesigned loads and manages several CSS stylesheets that change the appearance of popular tools.
(source: jason-saggers.com)
and some more 'unofficial' Gmail themes