They just appear too dark. At least on my computer. Overall not bad but just underexposed or darkened using Gimp. The best bet on all digital images is to try and get the exposure as close as you can. Then if you must, do some adjustments on the computer. Almost always when you try to do large adjustments on the computer the images suffer.
What I always suggest to people is to try and photograph the piece. If it's not coming out like you want send me the photo. then back off and shoot a photo of your photo booth showing me the lighting and describe what the lights are. I can almost always help you get a good photo if I see that.