^ As Force suggested, you'll need to make a larger banner to fit your width.
If it's a fluid (stretchable) style then perhaps a 3 part header will be required... gosh I been handing this link out like candy lately lol -
https://vborg.vbsupport.ru/showthread.php?t=228319
If it's a fixed (set size does not stretch to fit screen) width then simply make the banner the size of the main body of the forums.