this is a great idea. This is essential specially for image heavy notes. A big reason why I avoid to have too many images is because of this fact.
I also push things to a private repo in github for backup, but I also have my Notes of Drobox for quick syncing, since I only have a free 2GB storage, images are definetly a problem.
Even for Obsidian sync, I’m not sure what will end up being the storage limit, but having smaller size images would still be beneficial.
As a plugin
Of course not everyone will want their image qualities to downgrade, which is why ithink a plugin that allows the user to compress to a certain degree (50% compression, 25% compression) or even dimensions, that’d be great.