Enabling the ability to take screenshots in Morrowind is a quick and simple process. It only requires the editing of one line in Morrowind.ini.
You will need to open Morrowind.ini which is found in your game folder (default: C:\Program Files\Bethesda Softworks\Morrowind). Once you have it open, look for this line, it should be near the top:
Screen Shot Enable=0
Change the 0 to a 1, like this:
Screen Shot Enable=1
You will now be able to take screenshots in-game by pressing the Print Screen button on your keyboard. Any screenshot that you take will be saved in the game folder as Screenshot.bmp. You can also change the name that the game assigns to screenshots by changing this line in the Morrowind.ini:
Screen Shot Base Name=Screenshot
To whatever you prefer.
If you plan on uploading the screenshot, it is recommended that you change the file type from .bmp to either .jpg or .png. This is as simple as loading the screenshot in any image manipulation program (Paint will suffice), and saving it as a new image with one of the recommended file types. This is because an image saved as a .bmp could be several MB's in size, while the same image saved as one of the others could be as little as a couple hundred KB's.