how do i make paintings on minecraft properly?

you can cut down the break-and-replace loop by boxing in the space first. put blocks around a gap the exact size you want, say 2x2, and place it inside. it can only pick from paintings that fit, so the smaller the gap the fewer it has to choose from.

8 sticks around one wool, any colour. the part nobody tells you is you don't pick the painting, it's random based on how much wall space is free where you place it. so if you want one of the big ones, clear a big wall area first, place, and if you don't like it break it and place again. keep going until you get the one you want.