Custom banners have bad quality

[Aussie AU]

My custom banner (the one that first appears in my nations homepage) is 1300x400 and very good in quality but when I put it as a custom banner it has a very bad resolution. I opened the URL to the banner on nationstates.net and it was changed to a smaller resolution.

nationstates.net/images/banners/uploads/aussie_au_217551.jpg

[Eater of Cities]

Tried buffering?

[Aussie AU]

Updated OP. Looked at the URL of the banner and it appears it was automatically changed to a smaller resolution.

[Kelvaros Prime]

Maximum size is 1200x400 pixels.

Your uploaded image is 1300x400. Try cropping it a bit to a smaller size with a 4:1 aspect ratio and see if that helps.