After upgrading from Moodle 2.4 to Moodle 2.5 and also upgrading the course/format/grid to latest 2.5 branch (https://github.com/gjb2048/moodle-courseformat_grid/tree/MOODLE_25), the following issue started:
Updating the topic's title causes the grid image to be deleted. (Actually, change it's properties in the mdl_files table to 'draft' and rename it).
Workaround: Upload the image again, after topic title change. Or change its 'displayedimageindex' to '0' in the format_grid_icon table.
Note: The images disappear sporadically because there is a set cache time on the image, so the browser will cache the image until that expires. Which is why this is so difficult to spot because one action one day only becomes apparent the next.