Feature summary (what you would like to be able to do):

On export, I'd like the thumbnailed images that are PNGs to be re-coded as JPG. For many PNGs, this represents ~60-70% size reduction. While not so important on the website, on an e-reader, this can mount up quickly.

Simply adding ".jpg" to the thumbnailed image URL should generate a JPG automatically.

Ideally, there would be a user-selectable option to disable this so that users can choose to have the higher quality images if they wish (note that they'll still only get the size of image as thumbnailed on-wiki).