To pose for - To assume a position or attitude for a photograph or painting.
Context #1 – Two friends talking
Bob: I’m getting ready to pose for a photoshoot online. They want to feature me in a piece they are doing on successful business people.
Victoria: OK. Why are you standing that way? You look weird.
Bob: Well, the photos are of confident business people at work, so I’m standing confidently while I tell employees what to do. Also, I’m not smiling because I need an intense look to show control and strength.
Victoria: Hmmm…I think you are overdoing it a bit. Just be natural.
Bob: Victoria, you don’t understand. When you pose for a special photo, you must overact a little in order to project a certain image.
Victoria: If you say so. I still think you look like your shoes are on too tight!
Context #2 – An uncle and his niece are at a concert
Patty: Thank you so much, Uncle Sal, for bringing me to this concert. I love this band!
Uncle Sal: This band is good! I’m glad I came.
Patty: Come here and let’s take a selfie so that we can post it on Instagram.
Uncle Sal: OK. Wait, Patty. What’s wrong with your lips? They are puffy* all of a sudden. Are you having an allergic reaction?
Patty: Oh my god, Uncle Sal! Don’t you know that you are supposed to puff up** your lips when you pose for a selfie? Everyone does it.
Uncle Sal: Oh, I see. OK. Wait, let me practice posing for the picture.Patty: Ha! That’s perfect! One, two, three……
Meaning: "To pose for" means to make a certain posture or look for a photograph, painting, or drawing (usually a photo). In the first context, Bob is standing a certain way with a stern look on his face because he wants to look strong and business-like for the pictures. In the second context, Patty is making her lips puffy, or bigger, for the selfie and she tells her Uncle Sal that it is normal to do this for selfies.
*Pillow-like, bigger or swollen
**To swell or become larger