To upload a screenshot, click the "Upload a file" button:
Then locate where you saved the screenshot. (If you took a screenshot using the minecraft feature and you don't know how to find it, message me and I can help with that.)
The evidence should then appear for whoever reads the report.
"External URL Proof" means an image from a site that supports uploading pictures etc. An example of this, is imgur.com.
To use External proof, upload an image to imgur (or another website) then right click the picture and press "Copy Image address". Then press CTRL+V in the proof box.
If you have any questions or concerns, feel free to message a staff member.