Thanks @dd32 !
You’re right, it’s definitely a memory issue during upload relative to the image-size. I had tried a deletion and re-upload of one problematic file without making any change to it, and that confirmed it–same result. So then, resized it before re-upload which allowed wp to function, just fine. As an art-site, will need larger images now and then, but ftp’ing might be best for those needs.
Imagick checked out. Can work with the wp-config_php, for sure after a child-theme is set-up.
Thanks again!