Thread:Oscuritaforze/@comment-6604133-20150109060055/@comment-7697305-20150109065231

It's basically just as simple as tweaking the image in question to the right dimensions offline using image editing software (such as GIMP, Photoshop, or Paint.net). In order for it to (usually) fill the entire background space, you'd want an image that is somewhere in the vicinity of 1200-1920 px wide and 700-1080 px tall.

If you let me know which wiki it is, I can probably give help more specific to whichever image you have in mind there.