I've not got a Coppermine gallery but some of my sites were effected by the penguin update and have seen 500+ drops.
When the -amazon.com hack worked (it's now disabled) they would pop back to their previous page one places for the main keywords.
I'm working on the assumption that it's an over optimisation penalty, specifically your backlink anchor text profile. I'm creating content with generic backlinks to try to remedy this (with no luck as yet).
Try checking "opensiteexplorer" to check if your backlink anchor text profile to your gallery is "over optimised". If it is try to dilute it.
Don't know if it'll work but that's what I'm up to at the moment.