Thumbnails on folders show not correct

[Mar2zz]

When I select thumbnails in view when browsing my musicfolder it doesnt show a thumbnail correct or complete, but only the left/upper-quarter or above half of the picture inside the folder (e.g. see doors or dela soul in example-picture.) In every folder i have a folder.jpg of the complete front cover, that is the only picture inside the folder, in the folders reside more folders, with their own folder.jpg, same problem in those).

What is the source of this viewing problem, and where can i correct it?

example