impress images get lost from saved files

I find that after inserting jpg images into Impress slides everything is fine initially but when I return to a saved file most, but not all, the images seem to be lost. Each is replaced by a simple empty outline box of the same size that the image should be. In the top left corner of each empty box is a small icon that looks like mountains with a sun overhead. Also, next to the icon is a row of 3 dots.

I upgraded to LibreOffice Ver (x64) in the hope it would cure the problem, but it persists. OS = Win 10.0 (build 19041).

Can anyone please suggest what I might be doing wrong or if there is some setting I’ve over-looked?
Thanks, Mike