Hi @AndreaN , I believe the reason that your students are seeing errors when trying to upload images from the Canvas Student app is that it does not allow them to specify the location to save the files as My Files instead of Course Files, but they don't have permission to save to Course Files. You might wish to submit a feature idea saying students should be able to upload/save images to My Files when editing a page in the Student app ( How do I create a new idea in the Instructure Community? ), but those go slowly if at all.
The alternative you might consider for these journals is a group discussion connected to a group set where every student is alone in a group. Each student will see their own replies only. If students are allowed to add attachments to discussion replies (which is the default), they'll see the Attach button when composing a reply in the Canvas Student app, just like in browsers, and it works.