take a look at the page source, find the tag for the missing image .. what is the url it's trying to access for the image?

URL Master might indeed be interfering, not sure.  from the URL it looks like you have DNN installed in the "root" of your web server, is that correct?

the URL you pasted is

first this I would try is open my browser, enter the URL for the root DNN site and append that image URL to see if the image comes back and is displayed.

For example: if my DNN site's root url was http://www.gooddogs.com, I would enter the following address in my browser and I would hope to see the image displayed.

http://www.gooddogs.com/DesktopModules/Repository/MakeThumbnail.aspx?tabid=326&id=139&mid=909&w=100

If the image is displayed, then the problem is most likely something outside the module interfering with the URL (like URL Master).  If the image is not displayed, you might get back an error that might help you understand why the image is not displaying.

In the image URL, the id parameter, in your case 139, is the id of the repository item.  Use SQL Managements studio of Visual Studio's Server explorer to connect to the database and look at the grmRepositoryObjects table for the one with an id of 139 and check the image column to make sure it's pointing to an image that exists on your server.