You can easily replace a block background image on any device, even your smartphone. Here's how you can do that:
1. Navigate to the block which background you would like to change.
2. Tap on the empty space on the block to trigger the block editing menu.
3. Click on the "picture" icon. You will have options to take a new image with your camera, choose an image from your photo library, or locate a file to be used as a background. 
3. For this example, we would be choosing an image from the Photo Library. Click on this option and choose a suitable image from your device.
4. Crop the image using the preformatted grid which ensures that the image fits the block design in the best way:
5. Click Submit and the image will be uploaded to your website. Result:
Tip: to ensure that your image looks great on displays of all sizes and resolutions, including Retina, choose high-resolution images.