Menu → Posters → Add a picture. Bring a PNG, JPEG or WebP of any size; the game scales it to 512 across, saves it small, and keeps up to eight for you. A photograph that arrives as three megabytes is stored as about ten kilobytes, which is why a village full of boards costs nothing to walk through.
To put one up: choose Paint in the build palette, pick your picture from the Poster row, and click a side. A poster covers that side entirely — the pattern and the repeat size go with it, because a picture fitted to a face has nothing to tile. Choose none to go back to a bare wall.
Keep them decent. Anything indecent or hateful is taken down and the account that put it up goes with it. A picture is known by its contents, so taking one down blanks every board wearing it at once and the same file cannot be uploaded again by anybody.