What did I do wrong by adding a .jpeg ? player blanked out to all-white

Help/How To
3

I believe you will find it works better if you give the JPEGs their own separate track.

(if you need to move the video clips and the JPEGs together, when they are on separate tracks, @MusicalBox has made a nice little tutorial showing how. What do the three timeline buttons to the right of the magnet do? - #2 by MusicalBox)