There are no recommendations, the pictures can be scaled up to Full HD. Of course, you should pay attention to the file size that such 1920×1080 images are not equal to 10MB. Have you seen our demo so you can imagine it?
This always depends on the output device. But all images are scaled up to FULL HD. So the better the resolution, the better the quality, but more traffic.
Our editor is currently running at 720p, output on the device is up to 1080p possible, as it is scaled. When you insert an HD image into Editor, it will be scaled down to make it more comfortable to work, when output is scaled back to 1080p.
However, you can add a FULL HD image in each area, then it will be scaled down for output to fit the screen.