I've taken a couple of screenshots ingame pressing the "print screenshot" button of my keyboard and thus i've got some screenshot files with .bmp extension and i can't open them either on windows default image viewer and paint.net, i would like to know a way to see and edit my images.
Also i would like to regard that a week ago i had some issues trying to run the game but i got to a solution that unfortunaly i can't play the game with the GOG galaxy interface so i can only take screenshots with the ingame system

Ah, the bitmap ( .bmp) format... I have used the GIMP program to open it and other obscure picture formats (like .tga) and then saved it in another format, eg .jpg, but Paint should be able to open it as should Paint.net.
I checked and do not have Fallout 3 installed at the moment but maybe on the old hard drive. I could look, I guess. Then try to see what is up with that.

Now something is wrong there. Check your hard drive, memory and the game installation as I cannot see how it would put out pictures like that unless some files are broken.

I'm almost giving up the conventional way to take screenshots, i'm rather going to find a external software to take screenshots. Would you like to recommend me some so i could try out and see if i can get what i want?

Sorry, those are a big unknown to me.

I think there is something wrong with your installation.

I've ran the verify/repair on the game and tried taking another screenshot, the problem still persists. I don't know if all this is happening because of the modified fallout.ini file i'm using from the Intel HD bypass fix