How to Fix Common Macaw's Paintings Problems
Painting Won't Place
If right-clicking a wall does nothing, the wall likely lacks enough free space. Paintings need a solid block behind them and an open area in front. Remove torches, signs, buttons, or other blocks from the wall face and try again. Also make sure you are right-clicking the side of a solid block, not the top or bottom, and that you are not sneaking, which can change interaction behavior.
Painting Is the Wrong Size
Size is determined by available wall space at the moment of placement. If you get a small painting but wanted a large one, break it, clear more surrounding wall area, and replace it. If you get a large painting but wanted a small one, build blocks to shrink the free area to the size you want, then place again.
Can't Find a Specific Design
Since Macaw's paintings share the pool with vanilla paintings, you may cycle many times before seeing the one you want. Keep right-clicking the placed painting to cycle. If you lose track, break the painting and start fresh. Placing on a different wall can also change which sizes are available, giving you a different subset of designs to cycle through.
Paintings Look Dark
Add more light sources near the wall. Paintings are affected by light level like other blocks, so a dim room makes them hard to see. Place lights above or to the sides rather than directly in front.