It happens occasionally when opening xfdesktop-settings for the first time after session start or switching to another directory. A click on the preview area restores the missing/hidden thumbnails. xfdesktop 4.11.2git-369fe22
Created attachment 5375 screenshot
Created attachment 5396 Fix for images not appearing in settings app I finally found a way to recreate this bug. This patch should fix the issue. Let me know if it works for you since you encounter it more often than I do. Thanks!
I did several test runs and can confirm that your patch fixes the issue. Thanks
Thanks for reporting the issue and testing the fix. Pushed to master in: http://git.xfce.org/xfce/xfdesktop/commit/?id=565ccdfd80f0b4c6e6c67314224bb5c8128177c2