You need to copy the logo to the Photos app first.
If you don't want to bother with syncing through iTunes, you can email it to yourself (tap and hold in Mail then choose Save), or use an app like Photo-Sync (hyphen is important).
Once the photo is present you go to the Document Setup page,
select the header section you want the logo in, tap the Plus icon, choose the Media tab, then navigate to your logo and select it
You'll probably have to do a considerable amount of tweaking to get it exactly right, but thats the fun part.