tl;dr By reading some Internet and by performing own experiments I've learnt that no matter what file type my images has, PowerPoint 365 always saves them internally as PNG. Making file for any presentation with many photos extremely huge.
Is there anything I can do about it, beside using brute-force methods, like the one described here?
I have a 100+ slides presentation that has pure-text only slides and occupies less than 1 MB on my disk. I have added one photographic background to it. Even though source file has 360 kB on disk (saved as JPEG), PowerPoint presentation's file size jumped to 5,5 MB after that. After examining file contents I quickly learned that for some reason PowerPoint has saved this very single image as a PNG.
Is there anything I can do to prevent such idiotic behavior? This is my beginning into making full-sized photo backgrounds in PowerPoint slides. But I already have presentations like 30+ slides which (after saving to disk) has 100+ MB of size. An I am completely unable to send them via Gmail.