textures don’t have to be square. But their dimensions HAVE to be powers of two(like 128x256).
To put posters on the wall you have several choices. The easiest way to do it is just to draw another thin brush on top of your wall in whatever size you want the poster to be and then apply the poster texture to it. Then press S to open the surface inspector and press Fit to make the texture fit the brush. This method however may cause Z-fighting if especially if the poster is visible from a long distance or if the brush is too thin. But if the room is small , you should be fine.
Another way to do it is to do write a decal shader. This is basically just a shader with the keyword polygonoffset in it. It can be as short as two lines if you want…just polygonoffset and then implicitMap ….
The decal has the benefit that it wont cause any Z-fighting, and if you want you can “sink�your poster brush in your wall brush so that their surfaces are on the same plane, and both textures(wall and poster) will still be visible.
Or you can split your wall to form a poster brush, or do one of the methods suggested by the previous posts…whatever is easier for you.
I’m not sure what you mean here but let me just say that unless you want to go with the decal method, you don’t have to write a shader.