1080 x 960 seems to be qHD standard background size. I used a PC resampler/resizer program (Ifranview is a freeware one I use) to change some background images to this resolution and it seems to work well.
I just moved them to the Photon via the USB cable.
Settings/display/Wallpaper settings/gallery found them.
One of the odd things is that when you select an immage to use as a background, the select box frame does NOT select the entire image even if it is the proper resolution. I just touched the select and dragged the corners out to select the entire image and touch the "save" button. If you touch the selection frame just right it will get circles with arrows in them to be able to re-size it.
This would give you the best image use.
A strange bug thing is that sometimes when resizing the selection frame, it would do something strange and zoom way in. I would just hit the discard button and start over.
By the way Ifranview has many options under "color corrections" to make the image brighter or darker or to shift colors to personalize you image too.
XDA forum for the Atrix and the photon had a thread about background images, and had dozens already re-sized for qHD and ready to download.