As Mary has described, the messages are displaying as they should; there is nothing to be fixed here.
If you want more technical details:
When you look at the source code of an html-formatted message, you will see the text content of the message in among the html code, along with whatever html tags that reflect the formatting you've applied to the text (blod, bullet points, etc). Images are not in the message however; there is instead an html IMG tag that represents where the image is to be included in the mark-up of the message, and that IMG tag includes a source file reference, which in this case points to the Files area of the course on your Moodle system. Your mail program can't access that image file because it can't log in to your Moodle system, and it's certainly not enrolled in your course.
If you linked to an image source file that was somewhere on the open internet when putting together your message to send out from Moodle, instead of an image file that you
upload into Moodle, those images would display in the emailed version of the message, as there is no authentication needed to access them.