only one frame on timeline for images with not standard resolution.
here a video about that:
the project change dialog opens on project bin ... thumbnail correctly generated, but only one frame on timeline.
here the image for tests: http://www.wcn.it/testbmp.bmp
P.S. if the dialog opens on center of screen (know resolution/format) there is no thumbnail (and this is the issue: https://invent.kde.org/kde/kdenlive/issues/470 ) but the image as a standard lenght if placed on timeline.