By Subject Depicted

Breaking down the numbers,  ~38% of users use selfies/drawn images of themselves, ~21% use fictional or cultural characters, ~19% use images that are self-representative but otherwise unrecognizable, and ~13% use depictions of objects. (Note: the percentages don't quite add up because I rounded really poorly)

Selfies range from casual to staged and even complexly edited.

Self-representational images are often used by people uncomfortable with a very personal online persona, as well as those opting for a humorous effect.

Fictional characters are often used by users who want to display a favorite show or movie. Usually, they choose a character they feel very aligned with, so it still becomes a form of self-representation for others who are familiar with the character.

Images of objects are mostly used by blogs with a specific purpose or theme. They generally aren't personal blogs.