Resolution is the measurement of an image in pixels. Since the maximum available resolution of an image is only getting higher, digital cameras use a process known as 'Compression' to reduce the size of the image to a manageable size before it is stored. While this dramatically increases the amount of pictures you can store on each memory card, there is a trade off in quality.
Pending your usage of the images, the table below is a suggested guide.
|Usage of images
||Suggested Compression Mode
||Low - Medium