She is very cute, indeed. Without registration papers and a pedigree to view her past generations, it's very hard to confirm whether she's pure APBT
or mixed with something. Although, she does look very terrier type. You should ask the breeder for the registration papers. If he says there aren't any, than you can only guess that she's pure. This would fall under the classification that he/she is a BYB (back yard breeder); in other words, they're not on the up and up and just breed because they have a male and female, among other reasons. Regardless of where she came from, and whether she's purebred or not shouldn't matter. You love her, and she's a part of your family. Take the time to learn and raise her properly, and you won't be let down!
Just curious, though.. why did the breeder only show you pictures of her parents, instead of letting you see them in person!? That sounds suspicious to me right off the bat, but who am I to judge? Not trying to start an arguement here, just thinking out loud, so to speak. Good luck with her, and please do stick around as there is much to be learned here! Best of luck to you with her, and don't hesitate to ask any questions! The only question we can't answer, is the one you don't ask!