People should be encouraged to use alt text when they upload images to questions or answers.
Even experienced users with 110K rep have forgotten to do so at least one times.
A tooltip would be overkill, but a message on top of the preview box would be nice.
Update: Arjan suggests awarding a badge for adding custom alt text.
Well, I changed it from
alt text
to
enter image description here
and we'll see if that helps or not. Personally I doubt it, but the copy is better at least.
In my opinion alt-text on images is less about the preview box and more about accessibility, and for that it's important to know how to use alt-text properly. So while I agree alt-texts should be used more, it's also important to know how to do it.
Remember that empty alt texts (ie. <img alt="" src="image.png" />) are acceptable if the alternative content is repeated immediately before or after the image, or if the image does not serve any important semantic purposes.
The most frustrating thing, however, is that by default the filler 'alt text' is filled in the alt text blank for images, and more than 90% of the time this is left in the post after submitting. This is not to fault the post authors - coming up with a valid textual alternative to images is hard, and sometimes as notes above sometimes there's no need for a textual alternative for the images.
That said, having no alt text at all would arguably be better than having half a dozen 'alt text' images strewn across posts. The editor should, if possible, remove the filler text automatically when the post is submitted.
Requesting alt text in the image-upload GUI, as per this proposal, would be an effective and natural way to encourage people to enter meaningful alt text, especially if it comes with a well-crafted explanation of what is needed and why.
