You can also imagine she is fingering your own ass, just like in the picture.
But as far as I know, she would only do this if she's rewarded with a good fuck afterwards.
She will wishper:
"Spread your asscheeks dirty wanker, yes thats right, now i can see you dirty hole"
and moan:
*What a horny asshole you have down there. It's so greedy your hole. See like its tighten around my finger."
"So what do you want your mistress to do?"
Tell her: "Please fuck my dirty hole with your strapon mistress"