this tutorial is based on the Wallpaper and the Dynamic Wallpaper (via label element) tutorial. Actually once the initial wallpaper is set we can’t change it anymore but we can move the wallpaper image vertically via
set_skin_offset(). So if we use an image which is as double as high as our ui_size (in this case 2 x 608px) we can put 2 wallpapers into this one image file like shown on the left. The visible area is only from 0 pixel to 608 pixel.
We can now set a button for example which moves the background skin 608 pixel vertically to put the second wallpaper into the visible area. This short demo video shows how it works: