On the Accounts page on Polkadot-JS, in the Polkadot extension, and on many block explorers, small icons (also called identicons) appear next to your account address:

They are generated randomly when you create an account on Polkadot. If you don't like them or want to shake things up a little, you can change the icon theme on Polkadot-JS UI. 

How to Change the Style of Your DOT Icon on Polkadot-JS UI

1. Navigate to the Settings tab on Polkadot-JS UI:

2. Click on the "Default icon theme" to see a drop-down menu:

3. Select any theme by clicking on it. You can see an example of what the theme looks like here. We'll choose "RoboHash" in this example.

4. Click Save on the bottom right to apply the changes. 

5. Navigate back to the Accounts tab, and see your new RoboHash icons!