Sending images in a email can be more difficult than it seems, a few things need to be considered depending on your target audience.
The Email Template module from the AppStore can also send images, but uses Content ID (CID) which often doesn't work in web clients like gmail.
Replacing the Bootstrap RTE editor in the Email Template module (as suggested in other answer) will provide you with this option. The size of the email may become bigger, and often corporate email clients tend to block these emails although they do work better in clients like gmail compared to CID.
Linking to your image:
Linking the images in your email often works best when you need to send email to various different email clients. For example, if you have the option available, you can use a CDN (Content Delivery Network) to host them.
May be you could try the Rich text editor widget (https://appstore.home.mendix.com/link/app/16/Mendix/Rich-Text-Suite). That one seems to be able to insert images in your mail.
My mistake. The rich text suite is the old one that is superseeded by the CKEditor https://appstore.home.mendix.com/link/app/1715/Mendix/CKEditor-For-Mendix
That one you can style which button you can see.