Hey MC, I think I catch a bug.

Everything was working fine until today 2 items had the alternative color "1st" in the options list. The main picture will change to the LAST option of the list (even if is not the default). I thought that uploading the image for that option will fix the issue (since customer is not uploading picture for the attribute if the option is the main image); but that didn't fix it.

I also tried using the "default" button for the attribute, and doesn't work neither.

I can sen you by PM the links for you to take a look.