Digital photo resolution is defined by number of pixels of which it is made of. For example, 8MP photography has resolution of circa 3500x2300, that roughly gives amount of 8MP. According to that 5MP photography has resolution of 2560x1920. The relationship between edges can be 3:2 (3500x2300) or 4:3 (2560x1920), what depends on manufacturer and camera model.

Photography with 420x280 pixels resolution.

Photography with resolution 100x67 pixels on the 420x280 pixels size.