This is definitely a URL issue.
The correct URL of the Area51 'apple-touch-icon.png' should be:
and this is how it looks when you scale the width and height of the image: (
width="48" height="48" style="min-height:48px;width:48px")
Here is an excerpt from Google's Official Gmail Blog: (emphasis mine)
But thanks to new improvements in how Gmail handles images, you’ll
soon see all images displayed in your messages automatically across
desktop, iOS and Android. Instead of serving images directly from
their original external host servers, Gmail will now serve all images
through Google’s own secure proxy servers
Embedded images will now be saved by Google, and the e-mail content will be modified to display those images from Google’s cache, instead of from a third-party server.
As per the OP's raw HTML source code:
<img src="https://ci4.googleusercontent.com/proxy/rTOuR7U5y_NG8Vc7ok1XZvHOX4Ttaga6Qb9c5l5yPu2Z5iDrQpK3H1mJzKvyT-VZr-mbxqKHt3yDZ7o2Lgxi_FTkuRPU2tBwpWxH0hFr=s0-d-e1-ft#http://area51.stackexchange.com/img/apple-touch-icon.png" alt="Area 51" width="48" height="48" style="min-height:48px;width:48px" class="CToWUd">
You can see Google's proxy server is caching image from:
Whereas, the correct URL of Area51 icon image is:
Google's proxy server only caches/proxies image used in the SE's e-mail i.e. currently pointed to
http://area51.stackexchange.com/img/apple-touch-icon.png. So, the URL for the image in the e-mail is wrong as specified image does not exist at that address.
I am not sure when was the image from
area51.stackexchange.com/img/apple-touch-icon.png was moved to the current CDN location i.e. `cdn.sstatic.net/area51/img/apple-touch-icon.png'. I guessed the image location from the post "How does Stack Overflow make it so their apple-touch-icon doesn't have a border effect". All I did was replaced the 'stackoverflow' with 'area51' in the URL path.