it's the past participle of prendre. See e.g. http://www.french-linguistics.co.uk/verbs/table/prendre.html#.WtbfH8gh0w0
I'm not taking credit for this answer, it comes from BastouXII on a similiar sentence.
"here the direct object of the verb prendre (ai prise) is l', which stands for a woman mentioned in a deduced earlier sentence, and en photo is a circumstantial object of manner (how did I take her? en photo [In picture]). In English you take the picture and apply it to someone / something, in French you take the person (or the subject of your picture, whatever it is) and make a picture out of him / her / it."
Also from PRW15
It might help to think of the English idiom (at least pre-digital), "I captured her/it on film."
In this sentence, you take (capture) her on photo (camera).
I have given you a lingot for your research Suchiththa I really was wondering about "en photo" in this sentence because grammatically it does not make sense in English. the picture in English would be an object in this sentence ...so I was curious where the French" en" fits into the photo.thank you for your detailed answer. the Discussion forum is great.
It's not technically a forbidden liaison, but you would never make it to avoid ambiguity. So no, just because the next word begins with a vowel, does not mean a liaison is made (singular nouns, for instance, never liaise), especially if doing so would make the meaning ambiguous.