When you log into Live Blog, click on the user icon in the top right corner of the Live Blog interface.
Click on the profile link. This will take you to your profile page.
When you hover over the user icon, you will see an option to change the photo. Click on the CHANGE PHOTO button.
In the Change Avatar window, drag and drop a file from your computer or click on the Select from computer button to open a folder to browse from. The image should be at least 200x200 pixels. You can delete your current icon by clicking on the No image link in the bottom left corner of the window. Once deleted, you will not be able to recover the icon unless you have it saved locally on your computer.
Your Live Blog user icon is also your author avatar if the show author avatar checkbox is selected in the Theme settings.