I've been trying to display stuff in full screen mode from my mac on my television which accepts VGA input.
After plugging in the cable, going through setting up the display in system preferences, I was easily able to make the TV be a second screen sitting next to my macbook's built in lcd. So then I moved a preview.app window over to the television (drug it over using the mouse). I hit the slideshow button, expecting it to go to full screen mode on the tv.
But instead it went to fullscreen mode on the built-in LCD, and started showing the slideshow there, which was quite disappointing.
after some fiddling around, I tried moving the menu bar to the television. (in System Preferences, in “Displays”, go to the “Arrangement” tab, and there you can drag the menu bar to the external display).
it turns out it doesn't matter which screen the app is on; when it goes to fullscreen mode, it will display on whichever display has the menu bar.
I tried this in preview.app and also in Front Row. I assume the same behavior would happen in iPhoto too.