You must have heard about the long long looooooooooooooooooong discussion that has been going on about this.
For the "key content without alt text" cases, we have the alt="" attribute omitted, but there must be at least one of the following, and the first that is present must include sufficient information to orient the user.
- title="" attribute on the <img> itself
- <legend> of the <figure> that contains the <img>
- heading of the section that contains the <img>