To make it less "pixly", create an image twice larger and once rendered, reduce its size with an image processing program.
To generate nicer images, you may also define the two properties
com.eteks.sweethome3d.j3d.PhotoRenderer.lowQuality.antiAliasing.min and
com.eteks.sweethome3d.j3d.PhotoRenderer.lowQuality.antiAliasing.max
with higher values (see also this file). If you run Sweet Home 3D under Windows, that could be done by placing a text file named SweetHome3D.l4j.ini in program installation folder with the following content:
Hope you can do it [:)]