Here is the official Canvas documentation:

How do I embed an image in a discussion reply as a student?

Here's a more direct set of steps If you have a picture file on your computer that you'd like to upload into a discussion reply:

  1. Click Reply in the discussion
  2. Click the image icon (looks like a mountain) to upload an image from your computer
  3. If you wanted to use an image from your Canvas account files click on the arrow next to the image icon to display the following menu. A window will open. In that window browse to and select the picture file. 
  4. Once you've selected the picture file, click on the image in the rich content editor, select "Image Options" and from there you'll be able to type a description in the Alt text box (you can also adjust the image size), then click Update:

  5. Make other changes to your discussion post as needed, then click "Post Reply".