here's a way that may increase the percieved resolution of your custom background image.
(simply put) people perceive more vertical resolution (you can "see" more horizontal lines) than horizontal ("see" more vertical lines).
instead the common accepted dimetions of 300x170 with X:1.6,Y:1.6
my settings are:
254x200
(pixel width is X:1.88 and Y:1.36)
don't forget to use USM (unsharp mask) after resizing. I usually use r:0.6 @ 600% gain
http://www.alienminds.ca/PSP.bmp