Now you can because Google has opened up the Themes API.   If you want a little heads up so you don’t have to keep uploading and resizing the image look in your display settings of your control panel and find out what the resolution of your screen is.  If you make your image that size or as close to it as possible you will have yourself a nice new custom iGoogle page.

To get started go to http://www.google.com/ig/tm