My phone wallpaper is a photo of me and my friend's girlfriend.
What do you like about the photo?
She poses in the photo only in her underwear and its a sexy one
While it's great you are on good terms with your friend's girlfriend, it is especially suspicious that you have such an intimate photo with her. It is not healthy to have a sexy photo like that, because it can cause a lot of tension between you and your friend and their relationship.
I think it's arousing and I love it. It also makes me look like a bit of a player because I have a lady in my pocket.
What happens if your friend finds out? Then you'd lose your friend, and his girlfriend will probably think that you're creepy and stop talking with you too.