Change appearance of selected image

[Eike Kettner]

Hello,

I'm getting more and more used to view photos in Emacs. I now have a
buffer with thumbnails and when I move in that buffer the image is
selected as expected. However, the selected image is hardly visible as
being selected. A thin dark rectangle is drawn around it. How can I
change that? And how can I make Emacs draw/display another rectangle
around an image, so it may appear as “marked”?

Thanks! and kind regards
Eike