It turns out the behavior reported above only happens with the “shutter” thumbnail effect. When instead using eg. the “lightbox” effect, the navigation is exactly as i wanted it (viz. without the linking from singlepic images to random images).
Suddenly this has started to happen to me too. WordPress automatically inserts style=”display: none;” to images, (even in posts that used to look to perfectly fine up to now) resulting in images not showing. Anyone has a clue what’s going on?