We have been uploading and testing some images the Bitnami CanvasLMS stack and we could upload them to the "Files" folder of the course and preview them without problems. After that, we also created an announcement and included the file we uploaded previously and it appeared in the message we saved.
Some issues were reported in the past when using the Theme editor of Canvas LMS. You can find different workarounds in this community post but we also suggest you ask in the official forums of the application to get more information about that.
I hope this helps.