Find a picture online of something simple that your students will be able to make with a series of dots; an animal is usually a good choice to start with. Print the picture out and place it underneath a transparent piece of graph paper.
Create a scale on the graph paper using an X axis and a Y axis. Place dots on the graph paper on the outline of the picture at no less than 20 points on the image. The points you choose will need to be a points along the picture that your students will easily be able to create a picture, after plotting all the coordinates.
Find the coordinates of the each of the dots you marked on the graph and write them down using simple X,Y coordinates. Write these sets of coordinates out of order so students have to plot all the coordinates will have to be plotted before the picture can be identified.
On a piece of graph paper, write out the scale you used to create the drawing. Make copies for each of your students. Also make a copy of the sheet you wrote down your plotted X, Y points on. Give the students the paper and let them try and solve the puzzle.