In order to preserve appearance, you could use a vector graphics format, such as .svg or .eps, instead of .png which is a bitmap.
Also make sure you are using the right renderer if you go the vector graphics route (look at "Selecting a renderer" from the documentation). You should use Painters for vector formats.
How your bitmap looks like depends on a lot of things, the renderer, the resolution, etc... It can be a pain to deal with.