Why does he send me pictures of other girls?

Met a guy at party and we've been texting nonstop for 2 months since the time we met , it's been very flirty. We haven't texted for a few says then suddenly he texts and sends me a pic of him with another girl asking how he looks I said fine then he sends another pic of just a girl asking if I think she's pretty , apparently its because his mate fancies her. Why is he doing this and how do I deal with it?
Why does he send me pictures of other girls?
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