You can either upload an image, or upload a video to provide evidence.
Uploading an image:
Click the "image" icon, and paste a link to the pic.

Or, clicking "Upload a File" and insert an image from your computer.


Uploading a video:
Click the "media" icon, and insert the link to the video.

If the certain video link is not compatible with the media icon, just paste a link.

You have 24 hours to provide sufficient evidence.

Also

Please edit your report and use this forum to report a player for breaking any rule.
Please include:

1. Your ingame name:
2. The offender's exact ingame name:
3. A description of what rule they broke/how they broke it:
4. Evidence/full screenshots.