Before you upload something to e621, you're supposed to ask the artist for permission.
This can be tedious, so I've written a Firefox add-on that helps you.
(it only works on Furaffinity for now)

Download:
https://drive.google.com/file/d/0Bx_JbLEooF94NEUzZ0xKOElqa0U/view?usp=sharing

1) Open the .xpi in Firefox and click "Install"
2) A new icon with the text "AfP" is added to the Firefox toolbar (upper right corner)
3) Go to a Furaffinity post:
http://www.furaffinity.net/view/17835160/
4) And click on the AfP icon

Firefox opens a new tab with a new note.
The fields are already filled in for you.

Change the message if you want, and click "Post" to send the note.