So lately there have been people who wanted to show me a screenshot of someone breaking the rules, but didn't know how to upload it from their computer. This guide should be useful to them, and save me the trouble of trying to explain it to everyone individually.

Examples of confusion:
Sending links like this; C:\Users\username\AppData\Roaming\.minecraft\screenshots\2016-02-12_03.14.43 or file:///C:/Users/username/Pictures/download%20(1).jpe will not work. I cannot see them because they are only available to you locally, and they are not available to me through the internet.

Locating your screenshots:

Windows:
Photos are stored in this directory: C:\Users\yourusernamehere\AppData\Roaming\.minecraft\screenshots

Mac:
Photos are stored in this directory: /Users/YourUsernameHere/Library/Application Support/minecraft/screenshots/

Uploading a Screenshot to the internet:

Say that you're in-game and want to send a moderator a screenshot rather than creating a report on the forums. There are a number of ways to do this but I will suggest some simple ways that you can generate a link for your screenshot.

Imgur:

Gyazo/Lightshot/Puush:

Uploading a screenshot to the forums:

 

Thanks Wilbo! Very helpful