If wishing to use GIMP, this is how I've been doing it. I'm sure there are many ways.
1. Get GIMP
2. Get dds plug-in (Google)
3. Place "dds plug-in" application in a directory in
Local Disk\Program Files\GIMP 2\lib\gimp\2.0\plug-ins
(you may have to make "dds" folder within plug-ins with Make Folder, to maintain consistency with the other applications.)
4. Open your image with GIMP
5. Select the crop tool (4th tool second row)
6. Select "Fixed aspect ratio" and "1024x1024"
7. Left click and hold in the image, drag mouse to select the portion of the image you wish to keep. Reposition your square by click, hold and drag mouse.
Finalize your crop with pressing "Enter" key.
8. Under Image menu, select "Scale Image". Image resize by "Pixel" and "1024x1024". "X and Y" resolution" to "300 pixels/in". "Cubic interpolation". Press "scale".
9. Under File menu, select "Export as..." , and name it whatever you wish .dds.
10. "Click Select File Type (by extension)", and choose "DDS" image. (if DDS is not listed, check steps 2 and 3 above.)
11. Click Export and watch for the Save as DDS box to open. Under Compression, choose BC3/DXT5, and under Mipmaps choose Generate mipmaps. Click OK.
12. Go to Program Files (x86)\1c Game Studios\IL-2 Sturmovik Great Battles\data\graphics\Planes\"Your Plane you wish the picture to appear in"\Textures.
13. Rename the custom_photo in the Texture directory "custom_photo_original"
14. Place your brand new spankn' image here, and rename it custom_photo. NOT custom_photo.dds. Check properties on your new custom_photo. It should be
~1.3 MB, and a DDS file.
15. Just prior to spawning into IL2 with your chosen plane, hit the AP configure button lower right corner. Click the new 3rd button "Pilot Stuff", and click the photo.
The image icon will not depict your new photo. Start and admire your new cockpit photo.
Many theme and variations of this method work. Image below is in a G 14 I just made with the above.