Stackoverflow have supported markdown for a while. The syntax for showing an image is :
![Holes in Leaf 1] : http://i.stack.imgur.com/ECE47.jpg
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]] : http://i.stack.imgur.com/ECE47m.jpg : http://i.stack.imgur.com/ECE47.jpg