This is really a bit strange that they took a depiction which is described as a "Scythian noblewoman " in the book , when ,on the very next page in this book, there is a depiction of a woman described in the Text as "Scythian Queen ". Maybe the first picture was additionally published somewhere else as "queen" ? Tomyris isn´t even mentioned in context of the two pictures in the Osprey book. One other thing to note is, that if the picture in the Osprey book is the original they took their copy from, it is rather small ( under two inches high ), so they must have enlarged it quite a bit.
Obviously they didn't have the book itself. They just googled "queen tomyris" and voila there it is.
Edit: Wow, page 100!