Block posters is a very simple tool that allows you to turn a small photograph or classroom display into massive posters. It is achieved by simply slicing your smaller image into segments and then offering you the opportunity to print these slices out in a much larger A3 or A4 format.
Essentially depending on the quality of your original image you can blow up a a poster from anywhere from 4 to 24 times the original size.
Below are some visual examples of how it all works.
Block posters is totally free and you can access it here.