Are we talking about static theme, dymanic, animated window, Pro Slideshow??
If it's a static theme, and you are using the program you download from this site, I suggest using only 1920x1080 images at first. If they show up, then maybe the different resolutions were at fault.
I'm talking about a static theme and your suggestion doesn't work.
I'm using the PS3 Builder 4.0 Beta