How to PowerPoint 2013 compress background images (not just inserted pictures) to reduce filesize

I have a 10-slide presentation that is a whopping 234 MB!

This is because each of its slides has a different high-resolution image as its background.

(I know that the large filesize is due to these photos because I tested this by temporarily setting all slide backgrounds to a solid color, and the file size dropped to 1.5 MB.)

I've already tried the "Compress Pictures" feature mentioned here, but it seems not to affect background images.

I've searched all over the internet and have only found articles mentioning that approach and never mentioning the resolution of background images.

I'd really rather not need to manually resize large photos every time I'm about to use one as a slide background in Powerpoint.

UPDATE about why I care about this:

I think my Powerpoint presentations are the most beautiful I've ever seen because each slide has a different gorgeous photo as its background (with its brightness dimmed 40% so that white text is readable).

But these files are so big that my computer has difficulty running Powerpoint (even though I have a blazing fast new system with lots of RAM). Plus, I more quickly run out of local disk space or online storage space if the files are enormous.