Yeah, the idea came from an RP I'm in, where everypony turned to humans. Because they didn't wear any clothes, Rarity made some for the royalties. I design their clothes and figured that the princesses would fit on some modern greek dresses. Soon after that, we discussed about their cutie marks and where to place them. I did my research on google and I thought Princess Luna's large enough to be either on the thigh or arm. The arm seems more elegant and sexy. So I places it there. XD
Done with the usual Paint Tool SAI and Adobe Photoshop CS5.