Obviously when you find a really cool item or an awesome room filled with gold you gotta take a screenshot!
To screenshot in Diablo 1 all you need to do is press the "print screen" button. This saves a screenshot in the Diablo folder. The file type is .pcx which is very old and can not be opened by mspaint. To open it and save it as another file type you will need a program such as Photoshop or Gimp. Gimp is free and has most of the capabilities of Photoshop so I recommend using it!