Making the following assumptions, the fotos are portrets of people, so in general they are vertical. A very reasonable size for a picture would be a height of 800 pixels. For 3:2 this would give a pixelsize of 800 x 530 pixels.
A good quality picture would be about 100 kb to about 500 kb. And for very complex pictures the size would go up to 700 kb (for pictures with a lot of structures).
If you do not have control over the size of the pictures, 16 Mp pictures can take up far more space, this can take around 10 Mb voor a single picture.
So it does depend, but 500 kb for a picture gives enough quality for most pictures used on a screen.