I tried with the P2D renderer and the problem disappeared.
No need to add the smooth(8) line; the default behavior is smooth(2) and it should be enough.
If you can’t use the P2D renderer for some reasons, a workaround would be to create 2 or 3 versions of your image with photoshop or gimp to get different sizes and then depending of the size of the image you want to draw select the one that is the closest to avoid that problem.