Let's get first things first, it is OKAY to post pictures of someone who isn't your significant other on social media.
You know why? Because it shows that you have friends and a life outside of only being in a relationship and that person is your life and soul. When I look at someone's Instagram or Facebook, I shouldn't see pictures of either them or them with their significant other ONLY.
NO, STOP IT. THAT IS NOT COOL.
Post something of others to show you go out, you talk to other humans –– you know more than one person kind of thing. If you go around only posting the same person, people will get tired of looking at your social media, it will become boring and lame. No one wants their social media to be boring and lame, EVER.
A helpful tip to posting pictures with other guys/girls when being in a relationship is to make friends, and some ways to do is to become friends with those who are in your class. Specifically classes based on your major, because you will see those people for many years since you will most likely have the same classes together.
Another option could is to get a job, make friends with the people you work with, then BAM new friends instantly, and you see them every or most of the time that you work. Maybe you even have a picture of your family from many years ago, that can work just the same, it is someone in the end.
If you're a social person, a few options are to join a club, a sorority/fraternity, see someone sitting alone and you just walk up to them. Really anything is possible for social people. Maybe even download an app that allows you to talk to and make new friends, like bumble (pretty sure that's what it is, I am not one to use apps though, but know many that do and have been successful).
Something else that could be a possibility used is if your college has a Facebook page, you could post on it asking if anyone wants to grab a bite to eat or go get coffee someday. Anything is possible to make friends really if you try hard enough.
Now, that you have learned how to make friends because you need to add some new people to your social media to spice things up a bit; go out, make friends, be social, interact with other humans, even if it's an animal, and post a dang picture of you and someone else that isn't your significant other!
Also, always remember, NO MATTER WHAT, it is PERFECTLY OKAY to post ANYTHING to anywhere of anyone. Another thing, if you weren't dating this person, wouldn't there already be pictures of other people? Most likely yes, therefore it is still okay to post pictures of someone else even if you're in a relationship.