According to the "Stones people magazine n°12" the Stones tongue come from Kali , the hindu female godess :

"The actual idea of using the Kali tongue as raw model for the logo, however, came from no body else but Mick Jagger! He discussed the concept with an art director, named JOHN PASCHE, who did the final design"

There's a longue article about this in this n°12, where you can see a work of 1949, from 2 danish fellow, that is very close to the Stones tongue.But we don't now if this work have some influence on the Jagger idea and the Pasche's designed.

François

More on this - Keith in a Rolling Stone interview 1971:

...Especially now that we've got Rolling Stones Records, with the Kali tongue...nobody's got into that yet, but that's Kali, the Hindu female goddess. Five arms, a row of heads around her, a sabre in one hand, flames coming out of the other, she stands there, with her tongue out. But that's going to change. That symbol's not going to stay as it is. Sometimes it'll take up the whole label, maybe slowly it'll turn to a cock, I don't know yet.