Panorandom

picks a random panorama from all enabled resourcepacks to be displayed, instead of just the topmost one

the Mod Menu config allows you to change when it re-rolls the panorama, as well as allowing you to disable certain panoramas from showing up (eg the vanilla one)

Resourcepacks

a single resourcepack can have multiple panoramas, the mod will use any complete set of minecraft/textures/gui/title/background/[name]_[0-5].png

this means you could have files in a resourcepack named .../background/nether_0.png, .../background/nether_1.png etc and it can pick that as the panorama, or you could put them into a subfolder, so its ".../background/nether/panorama_0.png etc - as long as all numbers 0 to 5 are there it will count

if a panorama set has an associated overlay, then that overlay will be used alongside that panorama, otherwise the vanilla overlay will be used - so, .../background/nether_overlay.png will be used with .../background/nether_[0-5]