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.)

Step 1: Typical Instructable Photo

Can you discern what this photo is really about? My apologies to the author of the instructable for using their picture but.......

Step 2: Get the Gimp

Install the gimp and then load in the dark picture. (Most linux distros have the Gimp with a standard install)

Step 3: Now the Mod!

Go to Colors on the menu and choose Brightness Contrast.

Step 4: Do It to It and Your Done.

Increase the brightness and contrast. See the the difference? Press ok when you done and then re-save the picture. It's just that simple and remember the Gimp is a FREE program. By the way that was an excellent instructable where the picture came from. It also never hurts to have some extra lamps to have more light on the subject.

Note: you can only see the difference if you have a failing monitor. Updated picture with newer monitor.

Step 5: Fixing Schematics

Sometimes I download schematics and they are very hard to read. I found this quick trick to make things easier. If you do not have a program called the Gimp, download and install it. It's free!

Open up the picture with the Gimp. Go to "Colors" menu and then click on "Value invert". That is it, except for saving the picture.

Step 6: Removing the Squares.

Ever get an electronics diagram that has the checkerboard splotches? That is really a very easy fix. Load the picture into gimp. Go to <Layer> then <Transparency> then <Remove alpha channel> then click it and your done. Save the picture.
You may have to convert it to gray scale first.