I will second the suggestion at something like "expanded support for more image formats". One of my responsibilities is rolling the development log into customer release notes and I agree with the "changes that highlight a previous shortcoming can look bad", and make accommodations for that all the time. I also try to make sure every developer that contributed can recognize their work in the release notes.

"Expanded image format support" seems like something that if a customer hasn't noticed, they would assume "oh they must have some customer with a weird proprietary format that they added but have to be vague about". If it were related to customer requests, I would email the specific customers highlighting their need for webp is addressed after pushing the release notes