Thumbnail images do not appear in Gmail message

Symptom

When sending HTML email with a thumbnail splash screen embedded in the message, the thumbnail image does not appear when viewed by a Gmail recipient. This occurs regardless of Gmail's caching of the images and whether the recipient has clicked the "Display images below" link.

Cause

Gmail does not load images that are referenced using the protocol relative URL. 

Example: 

//play.vidyard.com/oTDMPlUv--51Th455G5u7Q.jpg

Solution

Specify a protocol when referencing the image.

Example:

<img src="http://play.vidyard.com/oTDMPlUv--51Th455G5u7Q.jpg" alt="Alt Image Text" width="360">