HI @SJN369
What you are describing is expected behavior. Canvas Inbox is a messaging system, not a full email client. For example, it doesn't have the full text editor functions that other pages in Canvas include. When you attach images they are treated the same as a document or video file would be, that is, a file to be downloaded.
If this is a message that you want to send to the everyone enrolled you might prefer to send an announcement which will allow you to insert an image with the text.
See the linked guide for Inbox attachments.
https://community.canvaslms.com/t5/Instructor-Guide/How-do-I-attach-a-file-to-a-message-as-an-instru...