Big Paintings

First, you gotta download the base Minecraft texture pack, then copy the textures folder into a new folder. After that you have to make a pack icon (256x256 pixels) and a manifest.json file in the same folder.

here's the template for the manifest! (use notepad or a similar app to make the manifest and change the file name to manifest.json) UUID generator: https://www.uuidgenerator.net/version4

{
"format_version": 2,
"header": {
"description": "Pack Description",
"name": "Pack Name",
"uuid": "UUID 1",
"version": [0, 0, 1],
"min_engine_version": [ minimum game version needed to run the texture pack (1, 17, 0 etc.) ]
},
"modules": [
{
"description": "Pack Description",
"type": "resources",
"uuid": "UUID 2",
"version": [0, 0, 1]
}
]
}


Then you go into textures > painting > kz.png. kz.png is the file with the painting textures. You can edit the file in basically any photo editing program. If you want to make the painting high res, you can scale the image from 256x256 to whatever size.
Then you can just place the pictures that you want onto the kz.png file. After that just save/export it as kz.png and replace the kz.png file in textures with the one you just made.

THEN you

hope this helps :)