How to make a picture lighter or darker
Make an image lighter/darker with GIMP
To make it lighter:
1) Open the image with GIMP (if not on disk yet, use this to get it from the web)
2) press Control-L (brings the layer menu)
...The "Layers" window appears
3) press Shift-Control-D or use the menus "layer->Duplicate Layer"
...In the Layer window a duplicate image appears above the first image and is selected as the working image
4) Select in the "Mode: [....]" control box, the mode "Screen" (see picture of the "Layers" window below)
...The image should be too bright, if not repeat 3 and 4
5) Use the slider "Opacity" to decrease the amount of brightness added to the image.
6) When satisfied with the result, use the menus "Image->Flatten Image" to merge all layers. (optional, a pop-up window will ask you the same thing in step 7)
7) Save the file with Control-S , or use the menu "File->Save as" to save under another name.
To make an image Darker, do as above, but set the mode to "Overlay" instead of screen.