In the previous rule, we suggested that parenthetic descriptions be used as alt texts for content images. This should be accompanied with some way of making the most essential part of information in the image available as text, naturally assuming that we cannot express that information conveniently in an alt text.
Explaining content images is often quite a challenge, perhaps too big a challenge. After all, we are referring to images that have rich graphic information. Quite often, it is not feasible to write a textual explanation. Content images often have more content that can reasonably be explained in text. For example, a news page contains news photos that are clearly content images, yet not necessary for understanding the text. You are not expected to do impossible or useless things. Yet, if some news photo contains essential information that is not expressed in the text, you should do something about it.
On the other hand, in many situations, textual explanations are both important and easy to write. For example, if the image presents an organization chart, you will usually need a lot of text to explain the organization with no reference to the image. However, it can be done, and it is rather straightforward. Imagine how you would explain the organization on the phone. On a web page, you are not limited to plain text, however. You can also use lists, tables, and other markup.
In a simple case, an organization chart would be explained in text as follows: “We have the board of executives on the top, with the managing director under it and overseeing all activities. The activities are: …”
Having written the textual explanation, choose between the following alternatives: