I have put the design on the lower right hand corner, (the napkin goes on the left, so might be covered up if you used left corner)when I was going to leave my table set with dishes for decoration, and I have put the design in the center when I was just using the placemat for decor. If what you want to use is very large, I'd go with the center, otherwise, it's your choice.;)

I have put designs in the center if rather large and it looks nice when plates are removed. Also on the upper left-hand corner. Also put designs on the upper right hand and placed a coaster just off the placemat above the design. It worked for me. So do your thing...go for it....