Some image files uploaded to the "course image" area of the "edit course settings" are not displayed in the block_myoverview (on the dashboard). We have tried with different PNG image files. Some display in the block correctly and others do not.
Technically, it seems like when an image is getting uploaded to the server, it's mime type is stored as image/png (mdl_files), while when it is getting loaded, its mime type is resolved as image/jpeg. This mimetype validation happens in is_valid_image and results in a default course image (random colored pattern) to be used in the block_myoverview