Okay, I get the above.
My problem is that I have a db entry for an image, a physical image and a physical thumbnail with the exact same rights and naming convention as my other images. But it still shows a line through ‘No Thumbnail’ when I try to add it to the post. It shows up when editing the post_parent and all.
I found out that an image of 523K does get resized correctly but my other image (700+ K) fails. If I resize it to a somewhat more modest scale it works fine.
Is the image thumbnail also database aware?