Once in a while we all have what would be the cutest photo, but
it's too dark due to backlighting or some other reason. One way
to lighten your photo in Photoshop involves the Shadow/Highlight
adjustment. Here is a photo where the subjects are dark due to
the window behind them.
Once your photo is open in photoshop, go to Image>Adjustments>Shadow/Highlight
The default setting is 50, which as you can see is a bit much.
Play with the sliders until you are happy with the way your photo
looks, then click OK. On this particular photo I used Shadow=20
While the photo looks better, you can still take it a step further
by applying your favorite photo filter or converting to black
and white. Here it is with a black and white conversion on it,
All in all, a fairly easy way to possibly salvage a too-dark
photo or two!