You can do this on a per-image basis using a bit of Javascript. It’s a little more work than Zap’s elegant solution (OK, it’s more than a little more work) but it’s pretty simple and it works great.
First, upload your full size image to wp. Either let wp make the thumbnail or create one yourself and upload it.
Second, insert the thumbnail into the post, and add this code around it:
<a href="URI of your full size image" onclick="window.open('URI of your full size image','popup','width=XXX,height=YYY,toolbar=no,directories=no,location=no,menubar=no,status=no,left=0,top=0'); return false"><img src="URI of your image thumbnail" border="0" height="thumbnail height" width="thumbnail width" alt="panorama" align="either left or right" /></a>
Obviously you’ll have to replace the “text” values above with the appropriate ones for your own site. I’m doing some testing with 2.0 at the moment, and I just tested this for my own use this afternoon. You can see it in action on my test blog in this post. You can view the page source to see how the values work on the test site.
hth