Some sites put the tag between [ ] like you have but this site uses html, and as pointed out above, in html you it between < >
For instance my sig is typed like this:
< img src= http://img38.photobucket.com/albums/v116/Image_Temple/willowface.jpg >
Now you can do other things like put a < center > tag in front of it to center it with any text you put beanth the picture. You can use the < font color="YOUR COLOR IN ALL CAPS" >Your Text Here< /font > this will color your text. And as pointed out above, none of the "tags" have spaces in front of the > or behind the <
A HTML tutorial may help you achive other styles you may be interested in. So look up some HTML if you are looking for a little more style control.