Best practices suggests that center sign visuals include real pictures. It also suggests that there be a picture of the number of children you allow in the center. (Math center allows 3 children, so there is a picture of three of your students represented on the sign)
You will notice that the top portion of the sign states the center name and 3 visuals of items found in the center. The bottom half of the sign is blank. This is for your to insert pictures of your students to represent the number of children allowed in the center. I would suggest that you also put "1,2,3,4" etc... under each child.