For some reason, iTunes 12 seems to lose artwork for videos. I’ve seen this occasionally, and heard from many readers who’ve encountered this issue. Today, it just happened to my iTunes library after I needed to re-install iTunes to troubleshoot a problem with my iPad.
When it happens, your movies look like the screenshot to the right.
The artwork is still there; for some reason, iTunes isn’t seeing it. To get it back, select a video, then press Command-I. You’ll see the Info window which displays the artwork. Forcing iTunes to read the tags of the file somehow gets it to read the artwork and display it correctly.
The only annoyance is the fact that you have to do this manually for each video; if you select multiple items, iTunes won’t read the artwork. So, click on one to select it, press Command-I, and then press Escape to dismiss the window, and continue this process for each of your videos where artwork isn’t displaying.