That’s a good idea you have. I make the sample project to prove a concept and help others to start with the work they have in mind.
in your case, you do not need to do anything other than save the canvas image directly to a Firebase key.
I have amended the demo project. If you remix it now you will see two new buttons; Save to Cloud and Get from Cloud.
Draw something in your canvas and then click Save to Cloud then click Get from Cloud to see the image you drawn showing again as a picture.
You can even exit the app and run it again and just click Get from Cloud and you should see the last saved canvas image.
Hope this could make a good start to your project.