Feature summary (what you would like to be able to do and where):
The exporter at wikisource: convert png images to jpeg or even gif (when png is transparent)

Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):

Dedicated ebook readers with limited storage space and display strength.

Benefits (why should this be implemented?):
I have been uploading jpeg because the destination is wikisource and perhaps ebook readers (often enough, my own). I would prefer to upload png as is the commons way. If the exporter would convert flat png to jpeg and transparent png to gif, I could resume uploading non-lossy png.