You can upload a custom icon into the icon picker by clicking the rightmost head icon or scrolling to the bottom of icon picker and clicking on "Add custom icons here"
In the next screen you can upload, name, and delete icons.
Before you upload, make sure your icon is saved in .png format, with transparent background. The file has to be less than 64KB in size and no larger than 512px X 512px. Also make sure it is black, this will let you color it any color you want later, when you put it inside an infographic.
Once you upload the icon, please refresh your browser window to see the icon appear in the Custom area in the icon picker.