Hey, I know this is an old thread but I am having this problem.
I thought I had updated all the relevant paths to full http URLs but the header image is still not showing up for both PhotoPost and for Classifieds.
The URL of my forum is www.aucado.us/forums
PhotoPost is here
Classifieds are here
I appreciate any help on this.
I worked this out. I had to put in the whole URL in the Header template.
All fine now.