Well as everyone has mentioned, the first step is the most important. That is to scan the image and save it in a not-destructive format like TIFF or PSD.
With the posted image, I copied the layer first, then applied a channel mixer adjustment layer. I used 100% Blue since that seemed to offer the best contrast.
I then used a levels adjustment layer to bring up contrast to the rider and masked out the "Dark" horse. I copied that adjustment layer to double the effect.
I finally copied the layer using a multiply blending mode and a mask to further darken the upper part of the picture