Why does adding a url attribute pointing to an image cause the email to stop working

We are on 12.1.3. I have a workflow notification message that I am sending with wf_notification.send. I have four url attributes that point to image files in $OA_MEDIA. I can see the images when I paste the urls into a browser. However, when I run the code to send the notification, the email never arrives. If I remove all of the url attributes for these four images, it works. Adding any one of them back in causes it to fail.

This was working previously, but after updating my code some and the notification some, it has stopped. So, I'm looking for ideas as to why they may not be working. What could I have done to cause it to fail. I've looked all over, but cannot find the problem (which probably means it's something simple that I'm overlooking).