Jake, this may have come up before... can we 'fudge' the attachment ID that the rebuild job starts from? Here's the scenario... under XF 1.5.x a different 'optimizer' product was being used, now with XF 2.x the site is using TH's optimizer Because some images have already been optimized they shouldn't be looked at again.By default it'll only optimize images uploaded after you installed the add-on. If you want to optimize all previously uploaded images you can go to Tools -> Rebuild caches and run the job from there. I'd recommend taking a backup first since it will touch all images. Nothing bad should happen as there are a lot of safeguards in place, but still a good idea
Thinking out loud, what is the actual logic for the rebuild job, if it doesn't exist in the TH queue table it tries to optimize it? What if we ran a query to populate the TH table from a select of the old product's table?