ReFit background image?
No. There is no option to change the background to an image, but you can set the color of the background to anything you like. 

This involves changing the 'banner' image (rEFIt logo by default). The background color will be set to the color of the top-left part of the banner image.

Changing your banner image involves changing the rEFIt configuration file. Instructions for this can be found at http://refit.sourceforge.net/doc/c3s3_config.html. Hope this helps.