So we had to disable it. I installed another plug in yesterday that does the same job. Images in posts will diplay not larger than 500 px, and you can click on top of them to open a larger version in another window.
Also, the upgrade seems to have reset the built in vBulletin file resizer. I adjusted the settings yesterday and now only images wider than 2000 px will be resized (but will still show up at 500 px in the posts).
Unfortunately, all the files uploaded between the upgrade and yesterday have been permanentely resized. If you want larger versions of thos epics, you'll have to reupload your attachments. But this shouldn't happen anymore from now on.
Also, as some may have noticed, I also have enabled the attachement thumbnails function: all images posted as attachment are now displayed as thumbnails (instead of links) and open in a lightbox when you click on them (with the option of opening full size if you click on the lightbox again)
We hope these solves all the issue and we apologize for the inconveniences.