We've had a couple of people ask for something like this, so we built an ActiveX control that you can download from this post to help you.
This control basically attaches what is called a pluggable protocol into your Internet Explorer session. You only need to refer to this once per webpage that needs to display an image stored in the AD.
To use the control, somewhere in your page you have something like this:
<OBJECT CLASSID="clsid:DE292439-102C-4D83-884F-34A2D37759B3" CODEBASE="AD%20Pictures%20Control%20CAB.cab#1,0,0,1" VIEWASTEXT>
<span>No AD Pictures image control</span>
</OBJECT>
Now that you have the pluggable protocol installed, you can use an HTML IMG tag as you normally would, except that instead of using http:// or file:// or whatever, you use excpp://.
The format of the URL for the excpp: protocol is {DistinguishedName}/{attributeName}, so you might have a line such as
<img src="excpp://CN=Test,CN=Users,DC=exclaimer,DC=local/photo" />
And that's about it. The ActiveX control is royalty free, but of course you will need to purchase AD Pictures to get the images into the AD in the first place. See our website www.ad-pictures.com for more details.