It's actually pretty easy to do in GarageBand, if you can figure out the interface. You can drag one song into the large area (to create a track). Then place the playhead at the end (like in iMovie) and import the second song after the first one, in the same track. You can cut out the gap, fade in and out where you like. Export the whole GB project to iTunes and it will be one iTunes "song" that you can select for use with your slideshow.