How many diy article photos do i see that are so dark as to be unusable. There is a free program called "The Gimp" (available at http://www.gimp.org/) which is available for most platforms. You do not need Adobe Photoshop to to this simple technique I am about to show you. Now that I have a newer monitor, those old dark pictures do not appear to be so dark anymore. Technique is still the same if needed.
Other gimp tricks at https://www.instructables.com/id/Enhancing-female-photos/.
Update: I changed the intent of the instructable. (removed a few steps for other content.)