Each block is a 1:1 aspect ratio. To make sure your photos look the best they can on all devices you will want to use a high pixel count on your photos.
For example: you have a photo that is 2 blocks by 2 blocks this is still a 1:1 ratio. You could have your image be as many pixels as possible to make sure they don't look fuzzy when they are on a bigger device. The less pixels you have the more likely it will be that your image looks blurry on a larger screen.
Commonly Used Aspect Ratios
Aspect Ratio | 1:5
Aspect Ratio | 2:3
Aspect Ratio | 2:5