The pigma pens work very well. Sometimes I can use a Sharpie with a thin point like a pen but be sure to it out first on your white fabric because Sharpies tend to bled onto the fabric before you can heat set it.
One more thing to think about, make sure that the pictures are drawn well within the fabric square so that there is a good seam allowance, I'd say at least 1 inch all around the square. You don't want to sew some of the picture into the seam allowance.