In the beginning I always had 2 or 3 pictures of different images I wanted when I went to the shop, I've always let the artist choose which, how big and where to put it..
He will never copy the image as is and will add his personal touch so an idea is better than a demand IMO.
Now I just walk in do a little talking and let Tycho do his magic, he knows what is best (and I never got disappointed)