If I use the MostlyCE-built in file manager to avoid the mosimage-glitch (see other post), I have stunning results.
First it works - almost perfect. I copied the pictures in images/stories to the new UserFiles/Images folder and was able to work with them. One nice aspect is, that - because UserFiles is a root folder - I can use the pictures for three different sites: A 4.6.5 site, a 4.8 upgrade and one Full 4.8 Install.
In all three sites I can select pictures from the image list. In the 4.6.5 site and the 4.8. Full Install I can use the FCK Filemanager without restrictions. In the 4.8. upgrade I only see the names of the pictures, not the thumbnails. If I open the pics in the editor, they show up. Integrating into Content works flawless.

Any ideas?