Go into My Computer --> Program Files --> Warcraft III --> Screenshots , then search your screenshot from there. (It is easier to search for it if you "Right Click --> Sort By --> Date Modified")
If you're wondering how to take screenshots, click on the "Print Screen" or "Prt Scrn" button on your keyboard (Mine's usually at the top-right corner).
To upload screenshots into your Hive Workshop's profile, User CP (At the top-right corner of your screen) --> Edit Avatar/Signature (Middle-left corner) --> Use Custom Avatar --> Under Option 2, click on Choose File --> My Computer --> Program Files --> Warcraft III --> Screenshots (Then select the latest one you took, by looking at its number)
If you meant taking screenshots from websites, Print Screen --> Paint --> Ctrl + V --> Save (Choose A Name) --> Follow The Above Step On Uploading Into The Hive Workshop)
Hope that helped!