George, the 'Gamma Correction' is in the 'Colour Corrections' section in 'Irfanview'.
Select the image you want to correct, then go to the tool bar at the top of the screen. Click on 'Image', and a drop-down screen will appear. In the 5th box from the top, you'll find 'Colour Corrections'; click on this, and another drop-down screen will appear, which will have two, smaller-sized versions of the same image, side by side, for 'Before' and 'After'.
Below these, on the right hand side of this screen, you'll see sliders for 'Contrast', 'Gamma Correction' and 'Saturation'.
Use the 'Gamma Correction' slider, watching the right hand of the two images, to adjust the overall brightness, density and saturation of the picture. Fine-tuning can be done using the 'Brightness' slider, to the left, and the 'Contrast' and 'Saturation' sliders on the right.
Experiment first, clicking the 'OK' button to see the altered image full size and, if happy, then 'Save' either as it is, or 'Save As' if you want a separate file, retaining the original in it's original state.
Note that none of the changes you make will be permanent until clicking one of the 'Save' options, so you can alter things as much as you like, without damaging the original, until 'saved'.
This is not always a perfect solution, but can vastly improve an incorrectly exposed image, or a poorly lit subject.
Hope this helps, and now, back to Paul's excellent thread.
Forgot to add - the Bechcraft AT-6 is the 'Texan II', and no wonder it looks like a Pilatus, or a Tucano, as both the Beechcraft and Embraer are licenced from the Pilatus design.