GIMP Tutorial

[denzjos]

Another workflow to lighten up a photo :
Make a copy from the original layer
On the copied layer :
Filters / Blur / Gaussian Blur (used here value 20 - if not satisfied, try different values)
Colours / Invert
Colours / Desaturate / Desaturate
Layer mode 'Overlay'
To lighten the photo more, make a duplicate from the desaturated layer