Nowadays, the main way to find out whether a person is dating / serious with someone is based on social media posts. Not the good old "relationship status" on Facebook, but rather Instagrams and Snapchats.
Posting an Instagram with a guy is like the step between "talking" and "dating" because its pretty much saying like "he's here to stay... for a while at least." Now all your followers--obscure and close--know you are unavailable.
Often, it comes with a lot of unfollows and blocks from the boys that were into you. Be prepared for the several text messages asking who he is and if you guys are official because you will be bombarded by them. Also, make sure it is a super cute picture because you know that picture is going to be screenshotted and sent through a bunch of group chats.
Unfortunately, prematurely posting pictures can throw a fork in the road, as he may be wondering whether you are taking the relationship too seriously too quickly or he will embrace the milestone in your relationship. But when you think about it, why would he have taken the picture in the first place? But then it is also like, why do you feel the need to share every aspect of your life; a picture might just be a picture.
It gets to be so bad, then people always assume you are dating every guy you a post a picture with. When all your friends are guys, you look like quite the player. There is no happy middle ground. What if you want to post a picture because you look good, regardless of who is in the picture?
What about when you decide to stalk your ex-boyfriend and see him posting pictures with the same girl? No questions asked: they are dating. Guaranteed that when they break up and you get the "I missed you" text he will deny ever having dating her or saying that she meant nothing.
The thing about social media relationships is that it is extremely tricky. There is no rule that indicates when it is socially acceptable to post. 1 month? 3 months? 6 months? You don't want to that girl -- and we all know when of them -- that posts a picture with every guy she has had a fling with, but you also don't need to be ashamed of the person you are with
P.S. If you don't know your status with someone, then you should directly ask them because odds are they don't know either. But that's a whole other conversation.
I think the main thing is to think less. If you want to post a picture, there might not be any meaning behind it. What does a follower's opinion truly mean? Also, there is this thing called the delete button. Yes, everyone has had the opportunity to see who you have been with, but relationships fail. Whether you want to keep a picture up, regardless of who is in it, or if you want to delete the picture to get rid of any constant reminder of your mistake, you have the option to choose how something is conveyed on social media.