Values from a mapped column (source column) will be merged as linked images.
Gmail message only contains a link to an image. The image can be hosted on your website or anywhere on the Internet. See comparison between embedded and linked images.
KEEP IN MIND: Values in the mapped column (source column) must be links to images you want to merge - in your Google Drive or anywhere on the Internet.
You can make images clickable and provide a link where it should take users after they clink on an image.
get LINK from a column
Each merged image from the selected column will be pointing to a link you provide in the link source column. This way you can merge an image which points to a different URL for each recipient.
All merged images from the selected column will be pointing to the same link you provide.