The menu-background is not able to be accessible by you, aa you are no supporter Also, try to make it yourself However, didn't it use the default skin (like always?)
Well You can't actually due to the way it's programmed You can only have one of the images at a time, and to change, you need to change skin/skin files for it to change (playfield.png cmiiw) As for the menu-background, it only works if you support osu
MenuBG is a supporter feature and it runs out when your supporter tag expires. You're welcomed to make your own skin and all the BG used during xmas seasons can be found in t/262187.