There are other ways of doing it.
With Gravit Designer, you can do basic photo manipulation, but Photoshop or Gimp are definitely better suited for handling raster images. Gravit Designer is a vector editor, meaning that everything you create in the application can be scaled and manipulated without losing quality. So, it’s best to import SVG or PDF files, as they are vector-based.
You can also create a mockup using vectors but it can be more time consuming.
The problem of using a photo is that you wouldn’t be able to perspective distort your images to fit the screen perspective.