Change the page toolbar thumbnail ?

Hey guys. I'm looking for a way to change the page toolbar thumbnails for each page, I'm talking about these : At first I thought it was the first image added to a page that would be used as thumbnail, but after some experimentation, I found out that it's not the case. Sometimes just by adding new images to a page, the thumbnail will randomly change to one of these new images without me doing anything. I have no idea how this thing work, and can't find any option to change it. So, here's my question : how do you decide which image will be used as thumbnail ? Thanks in advance.

I believe it is the largest image but sometimes it seems to not update correctly or it is using a broken image. Perhaps you have deleted an image without deleting it from that map and now it is using the broken image.

Yup, I just tried, it's definitely using the largest image, but I don't get how or when it is updating. Refreshing the campaign doesn't seem to do it, on some page it updated instantly, on others it doesn't seem to want to update at all. I'm puzzled.