I love the Boho look of henna tattoo designs and I wanted to put them on my pots.  So, I purchased a commercial license from an amazingly talented doodle artist, Jess Volinski.  To adhere the images on my pots, I print laser decals with the designs, then graphically design them on to each pot.  The iron oxide in the laser toner literally melts into the glaze like a stain in a 3rd "low" firing. That's why the designs look different on purple glaze, they look plum because of the chemical composition of the glaze mixing with the iron oxide.  It's very cool!  This process takes additional time to design each piece as well as an extra kiln firing but I think the result is worth the effort!  These designs are all permanent and will not wash or rub off.