I have thousands of old photos that preview can open, but I can’t upload them into the photo.app because the file format is wrong. I’m going to try and use SIPS to convert them all into png first and then add to photo.app. Thanks for this pointer.

Nice. After reading the man page, I see that it can be used to convert image file formats:

    sips -s format png photo.HEIC --out photo.png

or resizing:

    sips -z 300 600 original.jpg --out new.jpg

sips looks really cool, thanks for pointing this out.

Not gonna lie, I missed this because I thought it was related to macOS SIP, System Integrity Protection. Which is something I am deeply uninterested in.

That's cool. I wish it could convert webp images.