720X480 seems what is working. But your files should not be a meg. Way way too big. The slide show is meant to work with jpegs. I know you can create very dense, very large jpegs, but I'd take the color down to about 16 bit.
Files in the 100,000k and less seem to work best. I've also been able to display some other formats, but wouldn't rely upon them.
Remember there are a number of jpeg formats. Not all work with the viewer. I wish I could tell you which formats work, but I haven't been able to determine exactly which ones aren't being loaded.
If you want to convert different jpeg formats without loading a program like Corel Photo Paint, or Adobe, you can pick up Graphic Workshop on line from Alchemy Mindworks. It's a Canadian company that's been doing graphic conversion shareware for decades. You can get a working copy downloaded from their website. A great utility to have around if you are playing with graphic images.