It depends on how you link. Say that you have a picture stored on some other website. If you paste that URL into your message, you get a link that people can click to view the photo:
http://antiqueradio.org/art/Discovery.gif
If you enclose that URL in [IMG] [/IMG] tags, the photo shows up here in its native size, which may be big or small:
That's what results when you insert a photo using the Insert Image button at the top of the message editing box.
Displaying a super-giant photo in a message isn't a great idea. People with smaller screen resolution then have to scroll back and forth to read any text in messages on that page. For a very large photo, use the first method and give people a clickable link.
Phil Nelson