Everything in the computer world, including graphics, is based on the binary system. You can also think of the binary system being based on powers of 2 as the decimal system being based on powers of 10. The binary system is the simplest number system, since it only has two numbers.
Computers use the binary system because binary information is extremely easy to store.
A system of ON and OFF switches will do the job. Every 0 is marked by an OFF switch, and every 1 is marked by an ON switch. Binary digits are also very simple to work with mathematically, in fact, the computer can only do calculations on binary digits. Every time you calculate something on a computer using our standard decimal system, the computer converts it to binary, solves it, and converts it back to decimal and spits back out the answer. One might think that the binary system is very complicated, but it is not.