When you are replying to a discussion message or writing an entry for an assignment as a text submission, you are using the “rich content editor” in Canvas. There are a couple of ways to incorporate images into your content that you can find by searching the Canvas Guides.
But here are a couple of individual guides from Canvas that you can peruse.
- How do I insert an image in a discussion reply as a student?
- How do I insert course images into the Rich Content Editor using the Content Selector?
Both of these address the same question: How to get an image into your Canvas typewritten content (discussion, assignment text submission, etc.).