Stumbled onto this behaviour by chance.
When posting an ad you are given 5 lines to pick which files to upload.
Example 1
If you choose a file for the lines 1,3,4 (leave 2 blank), the first image will not show among the thumbnails. Instead its supersize link will be visible under the preview picture. It's preview picture can only be seen when you visit the ad. After you click on any of the other thumbs, you cannot get the preview of the first image back since there is no thumb for it. Apart from that the supersize link of that first image will stay displayed under the preview spot. Even when you choose other images, the link will still point to the supersize version of the first picture.
Live example:
http://forum.bmwslo.com/oglasi/showp...0&limit=recent Example 2
The interesting thing is that this does not happen when you upload only two files to lines 1 and 3 (also leaving 2 blank). In this case everything appears to be fine.
Live example:
http://forum.bmwslo.com/oglasi/showp...9&limit=recent Example 3
When you upload ony one image by putting it in line 2 or 3, the ad will be published but no picture will be visible.