alt (alternative) attribute is used when the image fails to load, or for user agents that can't display images (screen readers, very old mobile browsers, etc.)
According to the HTML5 specifications,
The most general rule to consider when writing alternative text is the following: the intent is that replacing every image with the text of its alt attribute not change the meaning of the page.
This means that the "image description" hint is actually not very accurate. If the image is not important (in the sense that describing it will not help readers), is purely decorative, or if its content is already described by the surrounding text, then the attribute should actually be left blank.
Whatever you do, do not leave the placeholder text in, since that is worse than leaving it blank.
If you need a tooltip, you can use the
title attribute. In Markdown, use
: http://example.com/image "Your Title Here"
![alt text](http://example.com/image "Your Title Here")
the same format is used for titles on links.