How to find the degree of variation between observers' ranking results? for instance the average correlation between observers

Let's say for example, there are 5 observers which are asked to rank from 1(worst) to 4(best) four pictures and the results are as follow:

Observer Picture 1 Picture 2 Picture 3 Picture 4
1 2 1 3 4
2 2 3 4 1
3 1 2 3 4
5 4 2 1 3