I reported this problem to support a month ago.
I find the first picture in the stack (the first uploaded)
tends to be the displayed one in the shop -
(or vica versa I cant remember which way it goes).
I suspect that it wont let me make a Render image into the default either.
SW - please explain how you can have about 5 different issues
with your product and shop image management
ongoing for months ....
Ordering products in shop inventory - sometimes works
uploading product images - usually works
setting default image - usually doesnt work
setting default image in shop - usually doesnt work
uploading new banner - works erratically
uploading new shop logo - seems to be fixed in last couple of days