Its not as simple as running any old application. Anything you add needs to be integrated into LMCE properly. This means you need to create a DCE device. You will probably need to create a slide show plugin for the core as well, which would work with the media plugin to receive the "playlist" MH Play Media command, and in turn send that playlist to the appropriate slide show DCE device on the appropriate MD - don't know much about how the plugin would have to work.
Then you will need to create a Device Template for the new slide show system, tell it which DCE commands to implement such as Play Media. You can use DCEGen to generate the C++ stub code. Then fill this in with the functionality you want. You could then wrap that around any tool you like such as feh to control it based on the commands coming from LMCE. But the feh tool needs to be able to receive new commands in real time telling it which image to display next, and send its own events telling the system when a previous image has finished displaying, so the media plugin knows to sequence the next image. Also, feh has to comply with certain requirements on screen/window management to be able to interact with the Orbiter in a way that does not screw up the window stack.
These last 2 requirements (sending/receiving commands/events in real time rather than batch of images to use, and the window stack thing) are likely to be the main issue for you. And may end up meaning you are better off writing the slide show code yourself as it probably can't handle these requirements...
