i use this generally:
<a href="http://img402.imageshack.us/img402/3599/arileen19072007bj4.jpg" target="_blank"><img src="//img402.imageshack.us/img402/3599/arileen19072007bj4.jpg" width="480" height="300" border="0"/></a>
a href="" point to the url where to show the full image
img scr="" it's the thumb preview that will show in your post
target="_blank" force the link (when clicking on the image/thumb here) to open a new tab or window
width="" height="" force the original image to show as thumb, change the size to match your original image ratio 480x300 works for a original image at 1440x900 for eg. or use a 320x240 for a 1280x1024 other eg.