Thanks for the quick response.
I created a custom provider MyLibraries and overwrote SaveContent as following.
(content.Thumbnails.Count == 0)
ImageHelper img =
 b = img.GetThumbnail();
Telerik.Cms.Engine.IThumbnail it =
(I know the name of the file in AddThumbnail is hard coded)
Now in the list (LibraryListView) the thumbnail still doesn’t show. When I manually request the url “…/MyLibraries/LibraryName/16640015.tmb.ashx?width=70&height=70&decreaseOnly=true” the thumbnail still returns as tif image although it was saved as jpg.
What am I missing?
Is it even possible that way?