Neal check your mail, mission accomplished.
Incidentally, I think your difficulties in making the correction were probably because the png file was in 'indexed colour' (which is where the palette of colours is limited to a specific range of tones, usually 256), needless to say this makes it difficult to make subtle corrections to things, so if whatever image editing program you are using can switch modes to RGB, it will be easier for you to do this kind of malarkey. Then, when you are done, you can put it back into indexed colour mode if need be. But be aware that this is not like flipping a switch however, and you do lose some quality if you flip modes like that a lot.
Chock