Images are an essential element of your personal/professional brand- specifically, the images you use to identify yourself or your company on social media. Often these are the profile pictures social media sites have you upload to your account (like in Facebook, Twitter, and Google+).
Using consistent images across all platforms allows the public to more readily identify you or your company wherever they come across you on the Internet. This does not mean that you must use the exact same picture on every social media site, but you should aim to have a common element that carries through all platforms and your personal/professional website. Most commonly this element is a personal picture (for individuals) or a logo or mascot (for businesses).
The first article linked below gives advice for choosing social media profile pictures, with an emphasis on professional individuals, while the second explains how businesses can use mascots on social media.
Why Your Social Media Profile Picture/Avatar is So Important: 7 Things to Consider
Knights, Pirates, and Trees Flock to Facebook