Dolphin 16.04 in Kubuntu Linux does not refresh thumbnails by pressing F5. I had two images A.png and B.png. I deleted B.png and renamed A.png to B.png. Now new image B.png shows thumbnail of deleted image B.png. I deleted ~/.thumbnails and ~/.kde/cache-mypc/icon-cache.kcache but this didn't help. There is very old bug report of same issue https://bugs.kde.org/show_bug.cgi?id=288691 that is in resolved status.
Works for me. What's your version of kio-extras? By the way, the thumbnails folder is ~/.cache/thumbnails/
I can confirm this behavior. For example a folder full of images. I open one image, crop it in Gimp, and save the file. The thumbnail in dolphin stays the same. If I hit F5 it still stays the same. Dolphin version 16.04.3
PS: kio-extras version is 4:16.04.3
Again, works for me. If I crop an image with gwenview, the thumbnail is immediately updated.
I have to correct myself. It works using Gimp and Gwenview but didn't work using "XnVievMP". The reason was that by default XnViewMP keeps the files date and time when saving. This setting can be changed so that the "last changed" attribute is updated to the current date and time and then the thumbnails are updated immediately. However I can not reproduce Karmo Rosentals problem which is probably unrelated, sorry.
Sounds like this is a works-for-me.