Hi Brion,
You might double-check in case the installer was mistaken or you edited the LocalSettings.php file by hand; try setting $wgUseImageMagick = false; and it will try to use the GD library functions instead.
This is what I did - I put "$wgUseImageMagick = false" and commented the following line out (the path to convert). The result is what we were looking for: thumb resizing works again with jpg and png.
Now I wonder: How did the problem begin? As I know (and checked in former installations), default is $wgUseImageMagick = false. I only activated ImageMagick when the trouble with png-thumbs began. The result was worse: we neither could use png nor jpg.
So turning back to default resolved everything... I can not understand the logic of that procedure but may be others can and so it may contribute to help others.
Thank you for your hints, Markus