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In my answer below:

Why does the ideal $(a+bi)$ have index $a^2+b^2$ in $\mathbb{Z}[i]$?

I used a picture by quanta from another answer.

I gave the link and the name, so I think that this is fine, but I want to check if everyone thinks that this is good practice.

Both the citation aspect and the practical aspect (can quanta change the image that I just linked or is it fixed once it is uploaded?) are of interest.

It is also true that my answer would be better served by a new picture without the little black arrows and with $a$ and $b$ not coprime, and you are welcome to comment on my laziness or just make a new picture for my post, but I would like to also have comments on the best practice regardless of this particular aspect.

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    $\begingroup$ As long as you've said you took it from quanta's answer, methinks you're fine and dandy. As for users being able to change pictures: no, that URL's somewhat permanent; the only way it can change is if the imgur server holding that particular picture goes kaput, or if the "stack" version (whose URL has stack in it) of that picture dies off (which has happened a number of times here.) $\endgroup$ – J. M. is a poor mathematician Oct 29 '11 at 14:08
  • $\begingroup$ Whether the image can change depends on whether it is uploaded to imgur (in which case it can't) or to some site in control of the original uploader (in which case it can). But as long as you use the image button in the toolbar (next to {}) when posting your answer (instead of copying the url), your image will be uploaded as a new copy to imgur, and so the image in your answer will never change. $\endgroup$ – ShreevatsaR Nov 16 '11 at 4:07
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Legally speaking, you can reuse and adapt any answer on Stack Exchange or any part of it anywhere, as long as you:

  • indicate the original site (if this is the same site, just saying so would suffice),
  • indicate the user name of original author,
  • link to the original post,
  • link to the author's profile,
  • and clearly state whether you're citing the post as-is or you've change something.

More precisely, as stated at the bottom of every page, posts on Stack Exchange are licensed under CC BY-SA with attribution required.

Ethically speaking, it's perfectly acceptable to reuse someone's work as long as you give credit. The legally required credit is sufficient.

Since the picture was uploaded to Stack Exchange's imgur site, it is (as far as I know) supposed to remain up as long as SE exists in its current form. The original uploader cannot edit or remove the image.

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    $\begingroup$ Sometimes imgur images die. But still, once the uploader has committed the image to imgur, he has no way of changing what's contained in the URL... $\endgroup$ – J. M. is a poor mathematician Nov 6 '11 at 1:10
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    $\begingroup$ @J.M. Images are now uploaded to a Stack Exchange server (stack.imgur.com) where images do not expire. Old images uploaded to the normal imgur site were transferred in a batch, and the image button in the editor now uploads to stack.imgur.com. $\endgroup$ – Gilles 'SO- stop being evil' Nov 6 '11 at 1:15
  • $\begingroup$ So link rot of this sort won't happen anymore? That's a relief... $\endgroup$ – J. M. is a poor mathematician Nov 6 '11 at 1:21
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    $\begingroup$ @J.M. that sort was a one time bug when SE switched to using the stack.imgur.com server instead of the old behaviour of just uploading to Imgur. The switch, as Gilles said, was precisely to prevent link rot in the long run. Of course, we cannot prevent users from linking to other resources that may be subject to link rot. But images uploaded through the add image dialogue on SE sites should be immune. $\endgroup$ – Willie Wong Nov 8 '11 at 9:13

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