It doesn't matter. The Ink app will go ahead and fit the photo to the card.
If you are using one of the "blank cards" which has no design and is "nothing but photo" the ideal dimensions would be 2100x1500 (or 2100x1500 if you are using the landscape layout card) or larger. You can use a photo smaller than this but as the size goes down below this the risk of compromised quality will increase.
Just as an example a 3 Megapixel camera produces images aproximately 2048x1536 (varies from camera to camera) so most modern camera phones SHOULD produce images of satisfactory resolution.