I can see my house from here!
Could it be that this function is currently inactive?
I recently uploaded a picture to a new game to the respective games gallery, and when clicking on "More Details" I only get a page saying "TEMPORARILY DISABLED".
Is there another way to add a representing image?
There is a trick to this If you open the image, and then click more info you get the message you mention. But if you go to "Browse Images" and then click the "More Info" under the thumbnail, you get the option to propose representative image.