You can insert images into the New Message Tool that reference a URL (but you can't insert an image directly from a machine, server, etc.). Want more flexibility with including images in your messages or to make the images actionable!?! Check out the Template Editor Tool and get as creative as you want!
Some churches accomplish this by having hidden pages on their website where they store images, or others use an online file system like Dropbox or Amazon Web Services S3. The URL must point directly to an image and not to a page that displays an image. Another option is to have someone on your IT team upload the image to your server and provide you with the link.
To add the image's URL to the Message, click the Insert Image Button.
If there is an image that's used often (perhaps a general email header), you could create a Template with that image already included so users just have to select the image URL (rather than tracking down the URL every time).