Custom Image tag following LinkedIn instructions not working

Question

I've followed the LinkedIn tutorial to a tee, contacted them and they said they would bring this to the engineering team's attention and to contact GTM themselves as well. I couldn't find a live chat button so now I'm here. 
First issue with the (directly copy and pasted) provided URL from LinkedIn after setting it up as a Custom Image tag:
"The URL must starts with //, http:// or https://"

Once I appended a "//" to the beginning, I attempted to publish the container. Then GTM says:

Am I doing anything wrong or is this simply LinkedIn negligence? I am leaning toward LinkedIn negligence simply because their live chat is extremely buggy - I had to initiate a live chat multiple times and one time their representative and I were disconnected "due to inactivity" WHILE SHE WAS TYPING.