In fact, this is exactly what i did the first time...:

### LATEST WIP ###
[ATTACH=CONFIG]image i want to appear[/ATTACH]
blah blah
[ATTACH=CONFIG]image i do not want[/ATTACH]
The thumbnail scraper updates itself after each edit. If i add a second image in my post (after an edit or a single preview), this image becomes the new thumbnail, even it is at the end of my post, far from the Latest Wip tag.

Incidentally, this doesn't work either when i upload images in reverse order.