It's very unlikely that you'll find any backgrounds tailored to the MX-780 screen size. I'm pretty sure that what Duct Tape is doing here is finding backgrounds he likes, scaling them to the appropriate size, then cropping out the exact pixel size he needs.

In any case, MX-780 backgrounds don't work the same way as backgrounds for MX-3000, MX-5000, or most other MX remotes, because the smaller processor doesn't support alpha transparency. It's possible to tinker with 780 graphics, of course, but it's not something URC really intended, so it's not as easy as it is with other MX remotes.