For adding images in the gmail, Send using HTML body
But the catch is - Images should be kept in a shared location and the URL should be embedded in the HTML
whoever have access to this image to the shared location - they can see the images. So make sure that the image location and file is shared to everyone in the organization
Use below site to generate HTML for your expected content. Hope it is accessible
then instead of the image URL - keep your image where it is hosted