I'm in the process of upgrade my 3.8.7 forum to 4.1.5 and when I view my images, I see all thumbnails fine, but the where the associated image should be, there's just a box with the image title.
No problem maybe an htaccess issue watermark issue without seeing the issue when it was happening its all a guess. Let us know if you need anything else.
Thumbnails but no pictures 
Linear Mode
