I messed with it for several hours last night and got some of the graphics files to appear in game. I did this by putting the files in the mod folder.
I put the campaign file and the json file in this folder path:
Age2HD\mods\XXX\resources\_common\campaign\
And I put the graphics files in this folder path:
Age2HD\mods\XXX\resources\_packages\custom-campaign-fallback\textures\
I made separate folders for the graphics called XXXbackground and XXXicons
Inside the json file, I set the below filepaths:
"MenuBannerFileName": "textures\\XXXicons\\banner.png",
"BackgroundImageFileName": "textures\\XXXbackground\\background.jpg",
"MouseUpBorderImageFileName": "textures\\XXXicons\\button_up.png",
"MouseDownBorderImageFileName": "textures\\XXXicons\\button_down.png",
"MouseOverBorderImageFileName": "textures\\XXXicons\\button_hover.png"
For whatever reason, it seems that in the json file, the filepaths assume \resources\_packages\custom-campaign-fallback\ before the URL begins. It doesn't make sense to me since the json file is in a different folder but doing this, I got most of the graphics to appear in game.
The exception is the banner.png file. That did not change for some reason.