Stackoverflow have supported markdown for a while. The syntax for showing an image is :

![Holes in Leaf 1][1]

After having a question edited I discovered that by adding a m to the image url you get the medium sized version for thumbnails. Link 1 becomes the thumbnail shown and wrap it up in a link syntax, when clicked you get link 5, the large version.

[![Holes in Leaf 1][1]][5]

[![Holes in Leaf 1][1]][5] [1]: [5]: