Make sure they all leave at least 1/2 inch around the edges for the seam allowance you don't want to sew into any designs they may have made up to the edges. By using the 1/2 inch they should go over and when sewing the 1/4 seam allowance you should be safe.
We did a mosaic using wild life animals and it turned out awesome!!! The school raffled it off and it was a success! Good Luck!