This issue may occur when the email triggers Gmail's spam protection filters. The cause of this can be different, however the following solution may work to get your email through:
- Edit your Email Content and click HTML to show the HTML Code.
- Find the code for your image. It should look similar to this: <img src="HTTP://www.blackbaud.com/imageurl" alt="Image Caption" />
- Change the HTTP to a lowercase http. It will look like this: <img src="http://www.blackbaud.com/imageurl" alt="Image Caption" />
- Save