I was wondering two things regarding what would prompt your UTR to send you that picture and then respond in the way she did.
First was whether your UTR has some issues where she is trying to show a hold on you more than just mere jealousy. Something along the lines of "Oh, no, you can't have her and me, too." Does she ever exhibit that type of dominance?
The second thought was that maybe this other girl is your UTR's "girlfriend." I've often read that providers are either bi or gay and have steady girlfriends.
You might see if the second scenario is true by testing your UTR. Tell her you would very much like to see her and this other girl getting it on together in a session with you. Promise that the other girl would be totally off limits to you and would just interact with the UTR. Again, I suggest doing this in person during a session right after you've handed her some cash so that she will be a better mood to consider it.