How do you fade an image using GIMP?

To generate a gradient of your image, use the Blend Tool, just left of the pencil. Click and hold the mouse on your image where the color should be darkest and drag it to indicate the direction for the gradient to fade in. To fade an entire image, put a solid white layer behind all the others. Then, in the Layers dialog, click the layer to be faded and lower its opacity as desired.