It's really easy to fix image quality problems such as over/under exposure or to brighten up dark images. You probably already have software on your computer that will do it at some level. In any case, here is a web-based application that seems to do a pretty good job with almost no effort. I put the same picture in half a dozen different utilities and this one was by far the best - even better than Photoshop Express online editor and Picasa. http://pixlr.com/express/
Click on Browse, select your photo, and edit. When you are done, click save and adjust the quality/file size balance. Usually something in the 50-70 range will be good. I'd give it a new name or save it in a different folder so that you maintain the original image in case you want to do something else with it.
The first picture below is the original image (I borrowed it from Mark here: http://www.pizzamaking.com/forum/index.php/topic,25127.msg285633.html#msg285633
The second picture is the result of clicking only three buttons: Adjustment --> Auto Fix --> Apply
There is lots of functionality to fine tune from there, but generally, it's probably not necessary.