[1.13+] List of custom Painting textures that can be applied by renaming painting item in an anvil and then putting it into an item frame to display it (The painting will have transparency).
Looking for a specific painting to rename? Use the search tool [strg+f] and type in the paintingname, matches will be highlighted.

Renameable Paintings

Wallcarpet

Painting

Large Wallcarpet

Painting

Large Curtain

Painting

Giant Map

Painting

Big Map

Painting

Wooden Crate

Painting

Wantedposter

Painting

Toolsketch

Painting

Wooden Taskboard

Painting

French Stonedecor

Painting

Small Map

Painting

White Blue Sigil

Painting

Red Banner

Painting

Blue Banner

Painting

Pots and Pans

Painting

Lord Portrait

Painting

Engineer Plan

Painting

Notes on Wall

Painting

Large Painting

Painting

Landscape Painting

Painting

Kitchen Knives

Painting

Ice Knight

Painting

Metal Doorknob

Painting

Castle Painting

Painting
[In case there are textures ingame that are not listed here, report them on the official Conquest_Community Discord.