comparing last and last before one rendered pictures

I have seen someone used this feature in blender, but I cannot find description about it.
It works (if I remember well) he rendered a picture, change something, render again, and there is a key combination which we can change between last 2 rendered picture.
Do you know anybody about it?

Render your picture. Press j - puts in buffer or something. Make changes render press j to switch between them.
That is J as in Jim