[development] Processing an image when a node is saved (D7)

I have a content type with an image field. I want to pull some metadata from the image file when the node is created or updated. I thought this would be pretty straightforward, but I'm baffled by some things that are going on in D7. I thought I'd implement hook_node_presave(). Apparently the image file isn't properly known by $node at that point. Krumo shows field_image['und'][0] but none of its child elements have useful information. And I have no idea what 'und' is all about. I added node_load() to see if that would help. On an existing node, it makes a difference. Krumo shows: