Step One: Open image in editor and select Rectangular selection tool. Press Ctrl+A to select whole image. Press Ctrl+C afterwards to copy the image to clipboard.
Step Two: Paste the image using Ctrl+V shortcut. Select Overlay mode in toolbar. To alter the effect strength modify Layer opacity. If the result doesn't satisfy you, try other blend modes or the alternative step which is described below.
Step Two (alternative): This screenshot shows result of auto levels (Shift+L). Contrast has improved, but colors are a bit dull. However that doesn't mean that one method is better than another - every image has its own needs. Result of this method can be improved using Vignetting function (Ctrl+Shift+V) with automatic mask (strength 50%) and by applying levels afterwards.
