Like @pafflick suggested, background-size is all that needs to be changed:
@nomadic said in Optimal resolution for Window Background Image?:
@Gumballer I did a bit of investigating, and it looks like the issue is because the image is being applied as a background to cover the whole browser window instead of just the header. That is why only the top part of the image is visible.
Inspecting the header, I found it to have a height of 39px, so with a simple CSS mod as @Ornorm suggested, you can get the desired result:
background-size: 100% 39px !important;
Note: I believe 39px should be standard across all instances of Vivaldi, but if the image is slightly off, you should adjust it to the height of your header.