But what do these all mean?

What does it show?

In my opinion, a profile picture is generally kept up to date with the most recent event that you got a nice image of. For example, if you took a good photo of yourself one day (whether it be at an event with friends, or just because you found some nice scenery/lighting, or just a random selfie that you liked), it could be a potential profile picture. Or maybe you went overseas and was excited to post your experiences. Some people decide to use baby photos as a "throwback" to back when they were kids, and some people use their graduation photos as a way of "celebrating." 

Filters are an interesting way of personalizing a photo and making it stand out as well as add another meaning to a photo. For example, just this summer, Facebook came up with a rainbow filter that would represent that you supported pride and the LGBTQ movement. This quickly went viral since marriage equality recently got passed and everyone was ecstatic to show their support for it. A grayscale filter is also a cool way to make your photo classier and subtly stand out.

On the other hand, a profile picture doesn't necessarily have to be a photo of yourself. For example, I remember when I was younger, I was too worried about posting a profile picture for privacy reasons so I would just use a picture of something cute instead as my profile picture. One of my friends constantly updates her profile picture with pictures of anime characters of her current favorite anime of the season, and another one of my friends just uses an anime character as his avatar representation of himself.

Others just straight up don't have a profile picture, whether it be for security/privacy reasons or laziness or ambiguity, there could be many reasons why a person could choose not to have one.

Regardless, a profile picture can help us learn more about a person, as it can transfer meaning besides just what you look like. It can tell a viewer what you've done recently, such as visit a foreign country or get a haircut. It can show a viewer what you support, such as LGBTQ, or what you like, such as a certain television show or character. It can advertise an event that you're hosting soon, or promote your sorority, etc.

A picture tells a thousand words.