After taking a screenshot in minecraft this is what you should do to find them.
1.Click the WindowsKey+R
2. It will pop up with a menu, here type %appdata%
3. Now you have a folder open, there you want to go to> .minecraft > screenshots
4. When inside the screenshots folder, go to the above bar next to the search bar, and copy that direction(?)
5. After this, go to the report site, add a screenshot and in the direction(?) bar paste the line of text you just copied.
6. It should bring you to the screenshots folder, and then just select the sreenshot you want to use.
If you have any further questions feel free to contact me!
Hope this helped!