Ok try right click the banner 'SAVE AS' watever you want. Put it on your Desktop as where to save it.
1. Now open the image using 'PAINT'!
2. When open go into 'IMAGE'
3. Now Select 'Sketch/Skew'
Change the 100% to what you want. Also if you mess up the size's just close 'PAINT' then it will ask you to save it 'CLICK NO' then open it up again and try again.
Then after you do that and get your size's right 'SAVE IT' then try using that as your Header!!!
Hope that helps a wee bit!

Nova