Yes, using the popup window setting, by itself, makes it more difficult to access the raw image. There is no address bar to see the URL, no menu to "View > Source". So, by disabling right click, you take away the last "easy" method. There is another code snippet to disable right click, but its specific to IE:
another <HEAD> tag:
document.oncontextmenu = function(){return false} or something like that.
Of course, these are defeated by a user by disabling Javascript, but then they can't get the popup. They can always right click on a thumbnail and determine its path and they figure out path of the _source or resized version.
A downside to the popup is that many cellphone browsers have weak support or none for javascript, so the gallery doesnt work for them.
There are other techniques that can be used. One is display image as a background of a cell and have a "clear" jpg be the <img> tag such as:
<tr>
<td background="myimage.jpg">
<img src="transparent.gif" width="100" height="50">
</td>
</tr>
and to do this in CSS file makes it even more obscure. The key is that its "obscure", not impossible to get the image. The only way to prevent that is to save the image as a myimage.resources file, then stream it out as JPG file upon demand. This cuts into page speed.
I believe there is also a way to configure the webserver (IIS) to deny the file or deliver an alternate file ("stop stealing my bandwidth!!") if the HTTP request is not coming from the same server. I don't know how to do this, perhaps its a HTTPModule type thing or advanced IIS setting. If this were known, then having a MySpace.com user set an image off of the site as their background would be halted.