Hi Sean,
Are subpages actually enabled for that namespace? The easiest way to check is to see if the breadcrumb link to the parent article is present on the top of the subpage.
Yes, they are enabled. I have enabled them for the main namespace and tested also with user pages where subpages are enabled by default.
I had a closer look at the generated HTML and found that both wiki links create the same HTML:
<img src="../wiki_uploads/5/51/My_picture.png" />
That relative path is wrong for a subpage as there's one path level missing. So I suppose the problem has something to do with my path settings.
Thanks for your help Frank