Somewhat accidentally I discovered an editing method to enhance the picture. Here is the step by step tutorial for GIMP:
1) Duplicate layer.
2) Run ‘Colors>Auto>Equalize’ for the top layer.
3) Switch layer mode to ‘Value’ for the top layer, and maybe 'Colors > Desaturate...' (turn monochrome).
4)Try to change the top layer opacity to find a pleasing amount.
This definitely does not work for all pictures but quite often it is a very quick method to enhance the contrasts and colors. Finally just merge the layers and do your usual magic.