Joseph,

I'm not sure that we can compare this photo with the PR work used by Obama's (or anyone else's for that matter) political campaigns.

This photo marked a turning point in propaganda - the time when photography became a 'player'.  This image is powerful, but I think the Lincoln towering over his generals in the setting was largely coincidental.  The PR and propaganda that politicians have used ever since that time have been calculated.

That's where I think the difference is.