@Troy I've run into issues in the past with camera phones having a large filesizes, but the lens itself just isn't up to par and provides a slightly grainy image. If you have an iPhone 6 PLUS or newer you might be able to get away with a camera phone but a proper camera is the way to go if at all possible.
If you don't have a suitable camera you are welcome to submit camera phone images but there is a 50/50 chance it may not be usable in the calendar!
ahhh ok
so the graininess
usually comes from low light situations.
Most modern cell phones take great pics in bright light, but suck ass when it's past 6pm.
(someone who knows more about cameras can probably 'shed some light' on the reasoning... as far as I understand, the graininess is the result of a bump in ISO [light sensitivity])