Background for Today Screen (not TF3D)

[the1zzisslow]

I'm trying to upload a picture for the background on my diamond, but every picture that I've tried won't center in the screen. If the picture is too big then it'll get cut off, or if its too small it will tile. I haven't been able to find any ways to select stretch option that will center the picture in my screen or is there a certain magical size that fits perfectly and I just need to adjust it to that size? Thanks for the help