BM has the right of it!
Instep is the bit twixt ankle joint and toes over which you would lace your trainers, (or in BM's case anchor the velcro straps). Some people refer to it as the "forefoot".
Beneath the foot the bit between heel and toes is the "arch" of the foot. People with "Flat feet" are said to suffer from "Fallen arches"
I would add that I would not have a tattoo there as I have once had blood taken from my foot, when the arm and hand veins just wouldn't give any and it was the most painful needle I have ever experienced.
Come to that I think it unlikely that I would have a tattoo anywhere.