Maybe this is a stupid question, but if a girl has a picture that is taken from behind her, showing a very inviting view of her butt, is that sign that she knows greek?

sounds like an invitation to me. i know i hope to express that view in some of my pics

NO!!! It means she has a better looking back side than she does a front side, or she just trying to be careful and nor show her face.  Usually, a lady will hide her face and try to show her best features and hide her worst.

Harley you on front me on back Greekinng it down the road.

sounds like a winner to me.. what about a cigar??
hugs 2 u deanna