Desktop image?

[Bob Goodwin <bobgoodwin wildblue net>]

On the desktop display [xfce] the background is an "F" with
what I suppose are bubbles.  I need to clear that space
to eliminate interference with the gkrellm display which
I run with a theme that provides a transparent background.

Can anyone tell me where to find the image file so that I
can change it?

Bob Goodwin