Let me preface this by saying that I am not sure if this an error with JRiver or my setup as I have also seen this issue with v19. I am currently using 20.0.10 on Win8.1 and all my files reside on an NAS.
When hovering over a random video file, that I know I have gotten "Movie & TV Info" for, the description will pop up, but without an image. Mind you that the Image File field is populated until the popup has come up without an image, at which point Image File changes to empty. I then look at the actual folder and, voila, there is no jpg for that file. It seems to happen at random, sometimes many files, sometimes only a few.
This has made it impossible to find which files are missing their artwork because MC believes there is an image. I have tried using image file=[] and various incarnations of ismissing(image file). Image file=[] doesn't work because I have to hover over a file before MC realizes there isn't an image file. And trying ismissing gives results that are inconsistent at best.
EDIT: While updating tags from Library, hoping to force MC to see Image File(or not), I noticed that the dialog box, once it loses focus, cannot stay on top of any windows. It always hides behind other open windows, no matter the program.