Each inline image has an abbreviation that, when inserted into your message between two asterisks (*) will result in the appropriate image being displayed in your message. For example, *g* will display the "Grin" image .
One point to remember is that there must be a space on either side of the asterisks surrounding the image abbreviation. For example, "smile *g* !" will work, but "smile*g*!" will not (note the missing spaces in the latter).
Visit the Formatting page and The Chip Board for a complete listing of the images available and their abbreviations. There you will also find other helpful formatting tips such as how to bold and italicize and how to create bulleted lists.