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.

http://i.imgur.com/5wFr5i1.png

Might as well as a feature to ask the commissioner/character owners as well

I gave it a try, it seems useful.

From experience of asking artists in the past, many of them have asked what e621 is or wanted reassurance that the posts will be adequately sourced and attributed. Extending the message to contain a little more information could be useful. Also, what's with all the linebreaks in the message?

Additionally, I think it'd be nice if I could run the script on their userpage, rather than on a post and request permission for all their art rather than a single post. I can imagine it getting quite annoying requesting permission for individual images, for both the person requesting and the artist being flooded with notes.

TheGreatWolfgang said:
Might as well as a feature to ask the commissioner/character owners as well

Seems far more difficult than it's worth, the poster's name can easily be grabbed from the page with simple regex because it's always going to be in the same part of the page. Since there's no standard way to announce the commissioner or character owners, it would have to use an awkward method of grabbing it from the description field.

Tuvalu said:
I gave it a try, it seems useful.

From experience of asking artists in the past, many of them have asked what e621 is or wanted reassurance that the posts will be adequately sourced and attributed. Extending the message to contain a little more information could be useful. Also, what's with all the linebreaks in the message?

Additionally, I think it'd be nice if I could run the script on their userpage, rather than on a post and request permission for all their art rather than a single post. I can imagine it getting quite annoying requesting permission for individual images, for both the person requesting and the artist being flooded with notes.

Seems far more difficult than it's worth, the poster's name can easily be grabbed from the page with simple regex because it's always going to be in the same part of the page. Since there's no standard way to announce the commissioner or character owners, it would have to use an awkward method of grabbing it from the description field.

Those are some good ideas, I'll add them next weekend.