Colors, Resolution & File Size

There is a trade-off between the quality of an image and its (file) size. Images with better resolution and more colors tend to be larger (more bytes). Larger images take longer to download.

A computer image is made up of pixels. The number of colors available in an image is determined by its color "depth": the number of bits of information devoted to each pixel in the image.
 
 

Color depth: n-bit GIF images
2-bit: allows 4 colors (2 squared) 
filesize: 4K
4-bit: allows 16 colors (2 to 4th) 
filesize: 6K
8-bit: allows 256 colors (2 to 8th) 
filesize: 14K
16-bit: allows thousands of colors (2 to 16th) 
filesize: 11 K
24-bit: allows millions of colors (2 to 24th) 
filesize: 26K