HTML doesn't offer as clean a solution for vertical lines as it does for horizontal lines -- a vertical equivalent to the Set your image up within your text and add a "style" attribute tag.

The Digital Inspiration weblog has a step-by-step for adding HTML to Gmail messages. For those of you who have tried inserting an image or table ... (give me plain text, please!), ...

Annoyingly, however, Word continues to insert images as “in line with text”–the one setting you’ll probably never want to use unless you’re inserting an image into a table cell.