Changing a PRIMARY POSTER

Hello.

I am trying to switch the PRIMARY POSTER for The Big Oil Alliance. I'm the producer. I've added the new poster to the image gallery and labelled it "Poster" but I can't seem to find the drop down where I can make it "Primary" as per the instruction in the help guide. Any assistance would be much appreciated. Our new poster is so much better!! Thanks.

Jeanie Calleja.