Sorry for the inconvenience. Really. And if it hadn’t been for these guys no changes would have been necessary…
Alas, they found a thing, and apparently people like Multisite and people make silly setups with it, so AdRotate had to be updated. There is no reliable way to update paths on the fly (during an update). So you have to do that yourself. Once finished, no further changes in future updates are required.
You mention tons of old adverts, you’re not using those. Just don’t update them.
Better yet, archive or delete those adverts. It’ll make your site faster, too.
The advert image is mostly relevant for current and future adverts of-course.