Other than PhotoShop which I love but requires a min to load with all it's plugins etc..
There is only one quick image editor with support for all formats etc..
That would be XnView, absolutely free and free of ads.. I can't pronounce the mans name that makes it but he knows his chit.. ')
I put it on every system I own and all I build for others as it also has extensions for file manager. Be sure to get the full ver.
Just do a Google on
xnview download and it will pop along with many excellent reviews of same.
After loading your image just goto IMAGE then LEVELS and pull in the right pointer, whala.. gl.
X/BillyD..