What is a 'Profile banner', and what is its file format?

[blekenbleu]

Something part way down 'Account details' page:

I suppose it will be obvious, once explained..

 

[Fat-Alfie]

It's a banner for your profile page

 

[Neilski]

Yes indeed, it's basically a background image I guess.
I don't use one myself but if you click the "Browse..." button, you may (depending on your browser perhaps) be given an indication of the permitted file types - see pic below for what I get.

 

[Bram Hengeveld]

It will add a background picture to the area i have marked in red

Dont mind my awesome art

 

[blekenbleu]

Dont mind my awesome art

Your art looks remarkably like my signature..
Thanks to all for the education.