Photoshop is an example of a graphics program that doesn't attempt to read the entire image into memory.

How much RAM did it actually use?

Difficult to say. I don't completely understand the activity monitor RAM column: http://i.imgur.com/QS3NPQQ.png Looking at the activity monitor details we see that it uses something in the order of 2.54 GB of real memory. I suspect the rest is mostly compression.

Tells you why it's a solid image application. Kudos to them.