Hi alde69,
The files you are trying to open in Photoshop from the texture folder are
extended Bitmaps which I think photoshop can't read - MSPaint certainly can't.
All you have to do is download
DXTBMP.exe - (Scroll to the botttom of the page). This is a program that can convert extended Bitmaps into 24 bit BMP image - which Photoshop can open.
The procedure below is how I can work with files in MSPaint - i'm assuming it will be roughly the same for Photoshop - although you may have some trial & error with converting to different files to find one Photoshop is happy with.
Load the original extended Bitmap into DXTBMP.exe -> Convert it to a 24 bit image -> Save as a new file -> open your new 24 bit image in photoshop -> hey presto! paint your plane.
When your happy with your artistic flair you just save the image again, convert it back (in DXTBMP.exe) to an extended bitmap (I have to select 'File type' DXT3) & then place that new image in your texture folder (using the original file name & overwriting the original file).
REMEMBER TO MAKE A BACK UP OF YOUR ORIGINAL FILE FIRST!!!!!!!
Gryshnak has a very good tutorial
here for repainting.
Welcome to SimV,
TSC.