Best format for audio track with a single/few images
The best compression would come from a variable frame rate format.  Variable Frame Rate formats are designed to adjust their frame rate based on the level of activity in the video.  For something with a very low effective framerate, this would allow it to store the image far fewer times and simply describe how long the image should stay up for.  It would then use more frames when necessary for any transitions.

The one downside to variable framerate formats is that there aren't a whole lot of well supported ones and even fewer good encoders to support working with them, but it is what would give the best results.

Handbrake appears to support VFR output.