Sorry for delaying... (those weekends tend to surprise one every unsuspected once and then ; ))
I was going to try briansthreadedcomments.php and iimage-browser. Since documentation to both of them is a bit scarce, I just throwed their respective downloaded folder-contents into /wp-content/plugins/ (next to the default ones (hello, markdown and textile), so this folder is populated with:
I throwed them into the folder _after_ the upgrade. As far as I can see, there is just this one folder wp-content, so my guess is that the location should be correct.
I'll go and check whether I had downloaded from an unofficial or something site (question of format etc.) but I think this should neither be the issue.