I'm not a very techy guy, but out of curiosity what does it matter? I know .pngs are usually smaller, but don't you want the background to be as high quality as possible? :O
I work with Inkscape that saves image files in png only. Because of this I was forced to convert to jpg files with Gimp and this work, do not know why, 'ruin' the graphics in the background when I install the app on my device (Galaxy Note), although the resolution remains the same (300 dpi / 5000x4455 px). I think the problem is the conversion from png to jpg ... but this is my guess.
One nice thing about .png files is that they support transparency and loss-less data compression. I use PSP7 for all image format conversions and post work, works like a charm for me.
