a pixel is not a little square

PIXEL
Unit 78 Digital Graphics For Computer Games
to understand the theory and application of digital graphics used for computer game 

Picture resolution: 

I refereed to http://www.microscope-microscope.org/imaging/image-resolution.htm during this section.

Resolution refers to the number of pixels in an image. Resolution is sometimes identified by the width and height of the image as well as the total number of pixels in the image. For example, an image that is 2548 pixels wide and 1836 pixels high contains 4678128 pixels you work this out by getting the width and the height then multiplying them and that's the amount of pixels in the image.

Image resolution:

Photo resolution is the quality of the image it depends on how many pixels are used in the image for example if the image is worse quality it will use less pixels we're as if its a high quality image 1920x1080 that would be the best of the best quality image other than 4k images. Also if for instance you're using a 22 inch monitor you would use 1680x1050 and if you are using a 10 inch notebook you would use 1024x600 

to make a photo realistic game or image you will need to use more pixels because more pixels mean more quality.The more pixels means more colour and more in depth pictures.  

there is over 16,000,000 shades of colour on photoshop which would easily create a 64 bit game which would look very real.