We automatically convert user generated images to WebP if feasible and then use the <picture> element to display alternatives (the original format as the fallback). I'm pretty sure that this might be a common pattern.
Browsers could allow a selection of all sources provided by the picture element (this would also include differently scaled variants based on media queries) in a download dialog.
The images are converted and stored when they are uploaded or changed. I can see this being an issue with that kind of volume though. But it’s a performance gain for the clients while content doesn’t need to worry about it.
Browsers could allow a selection of all sources provided by the picture element (this would also include differently scaled variants based on media queries) in a download dialog.