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The picture isn't that great to start with (no offense): only the middle two are properly exposed...the first one is too hot and the back one too shadowed. Use a second flash (and use a diffuser on the first one or something, or don't have them stand in a line like that if you're using flash) and use flashes powerful enough to light them all well and evenly.

Furthermore, the first lady looks like she just swallowed a turd.

All told, I say you should do this kind of thing in nice-looking available light or do really nice-looking flash.

But the scan looks fine -- why, do you have a print that looks better? It seems a little contrasty, but I'm assuming that's the lighting.