Here's hoping the opt-in is just a stopgap against slamming your servers with compression work and something can be implemented for primary images.
Could raise the settings a bit though. Going from 1.82mb to 183kb is nice, but the quality loss is visible. Could be mostly from the resolution...
Don't batch process. Do it on demand. This will skip a ton of older untouched content and prioritize new and popular material.
The first time a user pulls a page/image, check if it's been run through optimization. If it hasn't, check for and drop a meta file in to mark it as being in...