changing primary images

Hi there appears to be no way to change the primary image.  Trying to change the primary image for "Alpha Macro".  I had uploaded a photo just as a stand-in, and now it can't be undoen.  Under EDIT IMAGE TAGS, I see the phrase

"This is already the primary image. To suggest a new primary, please select a different image to replace it." 

But there is no way to replace it with the image I want (there are currently only two images and the wrong one is selected as primary image).