An image without a description is silent for a screen reader and for a search engine. Iris keeps the description of each image and reuses it wherever you insert it.
Writing the description
In an image's details panel, the alternative text field takes the description, then Save. Describe what the image brings to the text, not what can be seen pixel by pixel: "The old bridge under the snow" rather than "photo".
For a document, the same field becomes the link text: it shows instead of the file name.
The decorative image
A purely decorative image (a background, an ornament) should announce nothing. Tick No Description: Iris will insert it with an empty alternative text, which screen readers understand as "skip this".
The "to describe" badge
As long as an image has neither a description nor the box ticked, it carries the to describe badge, and its folder the red dot. The To describe search filter gathers them all.
On insertion
The Iris - Media editor button reuses the known description. An image not yet described does not go in unasked: Iris offers to describe it before inserting it (chapter 9).
Up to date for Iris 3.0.041.