Hi @slcooper,
I think you're adding the alt text to an image on a specific page (or other place) through the Canvas rich content editor, is that correct? If so, as far as I know that alt text will just apply to the place you set the alt text, it's not attached to the image file. It's possible that the same image would need different alt-text if used in different places, as alt-text should be contextual. The way alt-text works in HTML is that it does become part of page code, so Canvas would have to do a lot of extra work to have some sort of "universal" alt text for each image, remembering where that image is being used and changing it in all those places any time it's changed in a single place. It would be nice in some cases, but it wouldn't be a simple thing to implement.
-Chris