I tend to use GIMP to edit my photos and one of the things I have found useful is a quick and easy way to lighten shadows and darken highlights.
For those that know some of the terminology and have used GIMP (I don’t pretend to be an expert so bear with me if I describe some of it incorrectly) what I do is use a desaturated, inverted duplicate layer set to “soft light” with reduced opacity.
In more detail:-
When editing your image in GIMP go to the menu option “Layer” then click on “Duplicate Layer”. The duplicate layer will now be active but if you do something else before continuing you will need to click on the duplicate layer in the layers toolbar. We now turn it into a black and white image by going to the menu “Colours” and then click on “Desaturate”. I then usually select “Luminosity” and click on “Ok”. When you have done this a few times you might want to try other options. You can also adjust brightness and contrast but remember that this is supposed to be a quick and easy way of making some improvements to the image. You can get clever with layers and layer masks but if you are into that sort of thing you are beyond this little guide.
Now we can invert the image by going to “Colours” and then “Invert”. We should now have a black and white negative of the original image.
Now “all” we have to do is blend this with the colour original by going to the “Layers” dockable window. Press Ctrl and L if you can’t find it. In this window change the Mode to “Soft Light” and reduce the opacity to give you the image you want. You can turn the duplicate layer on and off to see the effect by clicking the “eye” in the check box next to the duplicate layer in the dockable Layer window.