One of the consistent challenges in this process is getting the picture into a useful format. By default, smartphones take extremely high-quality photos, and that means very large file sizes. It is not uncommon for students to have trouble uploading such a large file. In addition, an image file is often difficult for other people to annotate and comment on electronically. These issues can be solved, however, by using a scanning app to take the picture and immediately convert the image into a PDF. A PDF is generally a smaller file, and can more easily be electronically annotated and marked up.
Scanning apps are generally simple to learn and often available for free. One popular example of such apps is CamScanner. Students open CamScanner and use their phone to photograph their work, and CamScanner converts it to a PDF for easy upload to Canvas or elsewhere.
Another reason to encourage use of such an app is that they often have tools to solve common problems with photos of handwritten work. Two common issues that can occur when submitting photographs of coursework with a phone camera include underexposed backgrounds that make it hard to read text and the unwieldiness of having to upload multiple image files. Apps like CamScanner automatically adjust the color and brightness of the scanned file before upload, attempt to render any text as editable text rather than an image, and also provide the ability to create PDFs containing multiple images in one file.
As always, your particular course activities and approaches will guide whether and how you want students to use a scanning app for hand-written work. If you are using Harmonize image annotation, for example, then you will want students to upload the image rather than a PDF! But for Canvas assignments or quizzes, it will probably yield better results for both you and your students if they use a scanner app to convert their hand-written work to PDF for upload and comment.