You must first upload a picture to a server. Most if not all Internet Providers provide you with space on a server as a part of your service. Look around your Provider's web site for instructions on how to upload. Often you will need some basic software which will allow you to "FTP" files to the server. Ask your provider - you pay for their service so they should help you out. The answers are most likely already on a FAQ (frequently asked questions) area on your provider's web sit.
Once you have the photo on the server you can access it. There are two ways to access:
1. You can just type in the URL (address) and people can copy/paste it into their browser.
2. You can make a link in your message by clicking the link button which appears in the Format buttons above the area where you type in the text. When you click the button, the following will appear in the text entry window:
[*url][*/url] (disregard the asteriks - I had to put them there so the other text would appear)
Place the address of the picture between the two sets of URL brackets like so: [*url]your-link-address[*/url]
That will create a link to your picture which when clicked on will bring up the photo in a separate window (Like This)
http://www.pipeline.com/~randyo/m49/m49-1.jpg