This is one extension solution I have never seen, therefore it might not be possible to do. But it would be nice to be able to right-click on a message that came in and choose to “compress images”. I wouldn’t want it to auto-compress everything incoming since you may WANT someone to send you a raw file for something.
However, some mail clients default attached content as png vs jpeg, and those sending the emails may not be savvy enough to realize what they are doing. So getting emails that are 10-30MB in size because it has a few images attached and the person doesn’t know this is an issue, can quickly clutter an inbox with over-sized emails.
It would be nice to right click on an email or folder and say “compress” or “convert to jpeg” image attachments or something.
The best second-place option would be to right-click (or button) to strip images from an email. Instead of having to change the message body setting to list inserted images, delete them, then switch the message body back. That gets annoying.
There is an extension to strip images when sending emails yourself, which I do have an love. So when I reply to these emails, I hit that button and start typing my response. It’s useful for long conversations with too many images adding up.
So just need a solution for incoming mail coming from non-savvy users.
Just a thought 
Random included thought: There is nothing I like more when someone drops an image into their signature, sizes it down to fit, and sends without knowing how to properly do that. Then I get an email and wonder why it is 12-18MB in size realizing it’s coming from their signature image. gasp