Is the banner winter? I thought it was a spring thing due to the leaves or cherry blossoms or whatever on the tree on the left. Regardless, this is the path of the full image:

https://forums.totalwar.org/vb/image...ds/header2.png

The dimensions are 1,920px × 178px. 1920px width is required so that it properly scales for all common resolutions.

For the post background, do you mean this?
https://forums.totalwar.org/vb/image...unds/paper.png

If so, that's an image of the in-game paper or fabric. It's designed to match this top bit of each section/post/etc:
https://forums.totalwar.org/vb/image...ounds/flag.png

So, changing the 'paper' background would screw up the look of the skin unless the flag pole bit was also changed to match it.

If you'd like to create some alternatives, please go right ahead. I cannot guarantee that we will use them, but anything anyone produces will certainly be seriously considered.