@ redwing
what! give all my secrets away!!!

Technically it is 1x28, but I experimented and found that the border top is not stretched, like the BorderLeft/BorderRight elements, but is repeated, in blocks, across the screen.
So, after trial and error and info from TheaterMode that MB scaling is based on 1152x864, I made images that are tileable horizontally. Usually 384x28, but the polar bear fur needed more definition, so it's 576x28.
And that's how I do the "picture" borders.