It might be worth trying The Gimp or Paint.NET too. Picasa (by Google) is very good and I think that has brightness/contrast adjustment... even an 'I'm Feeling Lucky' button to auto adjust. All three are worth a try. You're only after a simple contrast/brightness adjustment really.
EDIT: It's a shame you don't have a program that will auto adjust the levels of your photos. This is the result of a copy and paste into Photoshop CS2, press auto-level, save as a 10% quality JPG:
It took about 2 mins to do from start to finish, including opening CS2, saving and uploading again. I'd happily do them in batches if you email me any pics you want me to do.
Last edited by Syph; 12-30-08 at 11:28 PM.