Hi,
The yellow question mark in the top left means the actual image is missing - you may have deleted/renamed or moved it by using Windows Explorer rather than within elements. The image may be on an external drive which now has a different drive letter.
The blue one in the middle and no thumbnail means that elements can not access the file (because it is missing) so that it can generate the thumbnail. This could mean the file was changed (as above) before the background task had time to generate the thumbnail.
You have turned off "Find missing files", why?
Your problem is caused by missing files so that does need correcting in some way.
Brian