I understand that this is linuxquestions and all, and I even run PN for my
website. I can't help but think that this is a post that really should have been made at PNs site?
Anyway, because I'm that type of guy. The quickest way to trouble shoot this problem is to right click on the where the image is supposed to be, get properties and find out, relative to your web server, where the image is supposed to be, then make sure that the image is there. Generally, with PN, if you have a customized theme, the place where you put your pictures is different than where PN puts pics.
Another thing to do is to switch the theme you use to see if it shows up under a different theme. Also, after looking at my banner, make sure that the way you've spelled it is correct in your Settings as well as the actual filename, upper case, lower case correct, etc. Also, make sure you don't have any spaces in the filename, things of that nature.
Post a link to your site, or update your profile to include a WWW for further help.