Created attachment 105122 [details] Thumbnails Thumbnails and Grid preview images still show old KDE4 screenshots of apps like Dolphin, KMail and others.
Created attachment 105123 [details] Breeze also shows the old screenshots
Breeze and Grid preview button shows the same KDE4 style screenshots on my 17.04 system.
screenshots from here https://cgit.kde.org/kwin.git/tree/kcmkwin/kwintabbox/thumbnails
Created attachment 105124 [details] New thumbnail screenshots
*** Bug 394285 has been marked as a duplicate of this bug. ***
This has since been done.