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Although the free and open source software  Raw Therapee is designed for processing raw camera images, it is possible to use its exposure correction tools to fix problems with JPEG images. As an example, this post demonstrates how to use Raw Therapee to correct a JPEG image for overexposure. Start Raw Therapee . The Raw Therapee application window appears . Use the File Browser , Places and Recent Folders panes on the left to browse and select a folder containing the photo(s) to be edited. Double click on  the image to open it for editing. The image is displayed in the application window . Observe the histogram on the right. The red, green, blue, and white curves are bunched to the right with a large spike at the end . Note: this is an indication of over exposure clipping . Under the Exposure pane, drag the Brightness slider to the left to reduce the brightness. At the same time, observe the histogram to ensure that the curves do not get clipped to the left. The im...

In Gimp or Photoshop , you can overlay an image with itself and blend the layers together with a blending mode e.g. multiply , dodge , burn , screen , etc. I found that a self-image blend i.e. an image with itself, with the screen mode can restore yellowed paper quite nicely and easily. By definition, the screen blend mode will lighten colors but leave black neutral and untouched. Using this technique, I managed to restore the yellowed paper scan below to a reasonably looking version quickly. Run Gimp . Open up the yellowed paper image. In the layers pane, right click on the yellowed paper image. A pop up menu appears . Choose Duplicate Layer . The layer is duplicated . Choose the duplicated layer. Then click the Mode combo box as shown below. In the drop down list, choose Screen . The screen blend mode is applied . The yellowed paper is restored and looks less aged .