Your photo is the centrepiece of the card, so it's worth getting right. A clear, well-lit image works far better than a dramatic but blurry action shot. Natural daylight or even indoor lighting without harsh shadows will do the job nicely. Aim for at least 1000 pixels wide — the higher the resolution, the sharper your printed card will be.
If you're photographing a young athlete, have them wear their kit and stand in a relaxed pose. A plain background (pitch, wall, or garden) keeps the focus on the player. Portrait orientation usually works best, though landscape can suit certain template styles.