The short answer is: "You can’t." Outlook (and several other email clients) enable you to include "stationery" or background images in your email message to "dress up" the look of your email messages. Currently, Gmail does not render these images as expected. Read on for some of my thoughts about stationery…
First off, I need to step up on my soapbox for a moment…
I really never understood the draw of background images or stationery in email messages. Other than allowing you to inject a bit of individuality into a message, what purpose does it really server? I personally find them typically distracting, not so much the images themselves, but the content chosen. Some are elegant and pleasing to look at, but some are just plain distracting.
I would suggest you take a step back and really think about what you are trying to do with email. Many people get so hung up with the style or look of a message that they often overlook the actual content. While it’s true that a picture can be worth a thousand words, well-written words should never be overlooked. And a nicely formatted and well-constructed signature can convey a lot of good information.
…OK, off of my soapbox.
Gmail does not currently handle stationery or background images, so if you really have to have it or use it, I suggest that you configure your Gmail account for POP access and then use an email client (like Outlook) that handles stationery. You lose Gmail’s excellent Labeling and Search functions, but you can continue to use an email client that you are familiar with, while taking advantage of Gmail’s excellent Spam filtering and storage space. You will still always have Web-based access should you need it.
If handling stationery is important to you, then I suggest you contact Google and recommend it as a suggested feature. Click on the Help link in the upper right corner of any Gmail screen, click the "Contact Us" link at the bottom, and click the "Suggest a feature" link. Google may not implement it, but if you don’t tell them what you want, they’ll never know!