Thanks for comments!
I made some further investigations:
Problem seems to be that the SONY Reader ignores some CSS comments in the EPUB file! It doesn't scale down images.
If a picture (JPG) in the EPUB is in its original size 1305px &
1331px CALIBRE overrides it in CSS with:
So far so good. But on the reader the picture is displayed in its original size (or at least try to do that) --> picture is too large --> picture is just partly visible.
So scaling on the PRS350 doesn't work properly.
The solution could be to scale down the pictures physically. With sometimes tons of pictures in a large book that's too much work.