Hello,
This is a bug report but just for the Windows edition so not sure if the post belongs here or not. I'm using v6.5.3. When I click on my library, bring up the little menu, if I choose Unpack Covers and then CANCEL, the program hoses all the covers in my entire library. It doesn't refresh them afterward. Umm, crap. Even if I remove the library, add it again, the covers are all missing and can't be refreshed. I'll have to delete the whole library and create it over again, which is going to take about an hour to load up all the covers again but that seems to be the only answer at this point.

This is very weird.

Please, create a little library (just a folder with a couple of comics) and 'choose Unpack Covers and then CANCEL'. If you loose your covers, open the ".yacreaderlibrary" folder inside the folder containing the little library and then check the covers folder, is there any file there?

EDIT: I can't reproduce this bug in any version of Windows (vista, 7 and 8 ) or MacOSX.
EDIT2: You can create a backup for your library (only tags+covers) using the "Pack covers" feature. If you have any problem with your library, you can save a lot of time restoring the backup.