Lately I've been using my "quick and dirty picture enhancement" trick quite often. Today I got an idea to combine it with the "gradient selection with quickmask" tutorial. So, here it goes:
- Duplicate layer.
- On bottom left corner of the picture area on GIMP there is so called 'quickmask' button. To use it, make sure you select all: 'Select > All'. Then click the quickmask button.
- Use 'Blend tool' with linear 'Gradient' to select the background.
- Click the quickmask button again, and the mask turns into selection. Now make sure you get the background selected. Click 'Select > Invert' if necessary to change the area selected.
- Run ‘Colors>Auto>Equalize’ for the top layer.
- Try to change the top layer opacity to find a pleasing amount.
As a result you should get more light to the dark background. Alternatively you can use round or other form of gradient selections depending on where your main subject is.