How self-contained should questions be?

Every once in a while, someone posts a question that relies on an image or a Wolfram|Alpha link with a formula or the like. In the case of images, it's often because they don't have sufficient privileges to include the image, and then someone soon includes it for them. In the case of formulas, it's probably because they want to save the effort of typing the formula out in $\TeX$ when you can already see it nicely typeset by following the link.

The main reason I see for this policy is that questions and answers are meant to persist and be useful for people who find them later, and we have no control over what other sites do to their URLs or even perhaps their continued existence. Some also emphasize in comments that it's annoying to have to click through to another page, but to me that's secondary; if it would save the OP a lot of typing, I wouldn't mind clicking through to another page. I'd like to hear your thoughts on this (in particular further reasons you may see for this policy), and to get an impression whether there's a broad consensus on this, since I think it only makes sense to ask people to adhere to it if there is.

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I completely agree. Part of the raison d'etre of this site is that questions useful for one are likely to be questions useful for many. In this spirit, it is a good thing to make Q/A as self-contained as possible. Note that in this spirit all images uploaded through the user interface are uploaded to an account that SE set up, which they pay money for, so that the image will not be deleted after a few months. –  Willie Wong Jul 27 '11 at 16:28
I think that some people don't know $\TeX$, or that it is possible to use it on the website, so posting images is not always out of "laziness". –  Asaf Karagila Jul 27 '11 at 16:29
Also, when the gender is unknown, I have seen many people use "they" instead of "he or she". (For future reference :-)) –  Asaf Karagila Jul 27 '11 at 16:30
–  Willie Wong Jul 27 '11 at 16:31
Some similar sentiments were also expressed in the homework FAQ. –  Willie Wong Jul 27 '11 at 16:31

Questions should be as self-contained as possible. SE questions are intended to be a long-term resource, analogous to Wikipedia articles, so they shouldn't rely on links that could easily become broken in the future.

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At some point one has no choice but to link. We cannot expect to duplicate here every mathematical resource that we cite. So it seems that what needs to be decided is where to reasonably draw the line. This may depend on diverse factors such as size of the excerpt, copyright issues, human factors (RSI), etc. It's a can of worms. Perhaps we should discourage images and links when it would be easy for the OP to duplicate the content here. But we cannot unilaterally exclude them because that would discriminate against those who have physical disabilities that make such duplication painful. –  Bill Dubuque Jul 27 '11 at 22:09
Sure. But if you need to cite so much material that it would be physically painful to reproduce it, I don't think your question is appropriate for math.SE anyway. –  Qiaochu Yuan Jul 27 '11 at 22:16
Does that mean that you think that there are limits on the complexity of allowed questions? Do you think a question about a long proof should require the OP to completely duplicate the proof, or link to it, or post an image excerpt? There are certainly questions that do require much context. In general I think it's better to include too much rather than too litle context (as do most questions). –  Bill Dubuque Jul 27 '11 at 22:22
@Bill: if you have a question about a long proof, you should ask about a step that you don't understand, giving enough detail so that at least the notation used in that step is clear, and perhaps link to a reasonably stable resource (so not Google books) for the rest of the proof if you think that'll help. I suppose you could reproduce a page-long proof if you wanted to, but most people aren't going to read it. –  Qiaochu Yuan Jul 27 '11 at 23:40