Up close sharp pictures are the absolute best.  With people, the face and torso work nicely.  Pets the whole body will work as long as the picutre is sharp.  

Sharp pictures with busy backgrounds will still work but look a bit off to me.  Grass is a great example of "busy".  This picture still came out good because the subject is so sharp and handsome.

This picture is old and a little blurry but still came out good.  The background is simple and a little photoshop cleared it up.  One of my favorite lithophanes.

Far away shots don't come out well in my opinion.  This picture is sharp, a little busy, but too far away.  People always send me pictures of 7 people dressed in white on the beach.  You know the picture.  I am working on a solution though.