How can I easily make the size/shape of a photo on my phone accurate to the proportions of a canvas when drawing with a grid?

So basically, I’m starting a new painting and I’m having trouble with matching the shape of the photo on my phone with the shape of the canvas. I’m curious as to whether or not there are any tools that can help me with this like an app or a photo editor. I am using a 16 by 20 inch canvas and I like to draw a grid to help with proportions but then I become worried that the image size will be off compared to the size of the canvas which would mess of the proportions. If someone has advice for this it would be greatly appreciated