Here's what I've got so far
Support for the CABINETS, CPANEL, FLYERS, MARQUEES, SNAP and TITLES folders (these are all pretty standard folders in the MAME folder structure to store related artwork. in each folder, you'll have an image file (most are PNGs though all the normal image files are supported, including ".JPG", ".GIF", ".TIF", ".PNG", ".BMP").)
Further, if a folder contains additional files related to a rom, those will be picked up as well.
for instance, say the SNAPS folder contains firefox.png, and ALSO contains firefox-a.png, firefox-somethingelse.png and firefox-otherimage.gif. They'll all be loaded into the slideshow cycle.
This will work for all the folders (Snaps, marquees, etc). You can also set which folders you want to include in the rotation (only snaps, snaps and marquees, etc).
The only snags right now are
1) getting the coords of the window right when the mala screen orientation is flipped or switched.
2) rotating the images themselves properly.
Just me being a tad dim when it comes to graphics manipulations in .net really (and not having tons of time to spend on it).
But I'll post back when I've got something to beta if you're interested.