I accidentally pressed delete, rather than remove from collection

Not in a staightforward manner. I think people usually use Webtools-NG for this sort of thing. I cannot provide any assistance with that, as I have never used it.

I guess if you recover to a previous database, turn off the Automatically Empty Trash After Scan, and then do a scan of your libraries, you will get the posters with the trash icon for movies that it does not find.