How do I display an icon on my profile and next to my posts?

You can add an icon or "avatar" to be displayed with all your posts. To add an icon go to "My Account" and click the "Edit Account Settings" tab. You can upload an icon and select it to be displayed.

Icons can be .GIF or .JPG files only with maximum file size of 10 KB. Maximum dimensions are 95 pixels wide by 95 pixels high. If a larger image is uploaded, it will be shrunk to this size. Images whose dimensions are smaller than these values will not be expanded.

Just like your signature this changes on all your posts - including past ones - every time you change it.