CiviMail Images always appear broken on screen after uploading correctly
If your images are being stored in a directory that prohibits direct access, it will give you that symptom. There's a .htaccess file that performs this function for apache, I'm not sure how that works with MS servers.

Your best bet is to try loading the image directly (e.g. right click on the broken image and load the image in a new tab) and see what the path/url is, and look in your server logs, it should tell you why that image is not being loaded, i.e. either whether it is a permission or just a not-found issue.