Instead of adjusting the brightness and contrast have a look for a 'levels' options. This will allow you to tweak the high, mid, low (and everything in-between) values. Most of the time you just need to tweak the high (bright) section and a little of the mid range.
Also take the screenshots at the highest possible screen size and visual quality you can. You can always scale down a shot and this will make it look 'sharper' too. Frame rate does not matter as you are taking a screenshot not playing the game :]