I added the Picture Frame applet to the desktop and configured an image folder but it shows no images. Instead of an image there is only transparency. Click on the Picture Frame to open the current image works fine though.
Probably the Picture Frame applet (https://userbase.kde.org/Plasma/PictureFrame) is not ported yet.
The plasma 5 has the Media Frame - https://git.reviewboard.kde.org/r/126793/ Working fine, at here, plasma 5.11.5: https://imgur.com/j97xV6K
Oh, you are right. On my other machine it also works. I have to check what is wrong with the other one.
I don't see this issue. Looks like it got fixed since being reported.