The photo of a manta is not downloaded until you look at it's description and details by tapping on that manta in the list. A small thumbnail is then downloaded and stored on your device for future reference. This also means that you can view that photo even when you are not connected to the internet.
You can also tap on the small image to download the originally submitted larger image of the manta and then "pinch and zoom" in order to get a better look at the details. Tap on the right arrow to find out if there are more images of the manta available. And then again you can tap on the image to downloaded the original full resolution version.