Go to the Object Tab, and click Object Export Options
In the Export Options, you will have the option to create Alternative Text. Change the Alt Text Source to Custom. In less than 150 characters, give a short description of your picture. Use the Custom Alt Text Source option.
Organize your text and photo so that the information is easily readable and your picture is easy to see and does not merge or mess up the text.
You need to make sure that the alt text is read at the right location. In the upper right area of the image is a blue anchor square. Click the small square and drag it to the document where you want the alt text to be read.
What about Decorative Images?
If the image conveys no meaning of interest to screen reader users, or if the image duplicates the content in the text, you should mark it as an "artifact," which is the equivalent of marking an image as (alt="") in HTML. Images marked as artifacts will be completely ignored by screen readers as if they weren't there at all, which is what you want in this case.
To designate an image as an artifact, select the Object Export Options, then go to the Tagged PDF tag.
Section 5.2: Complex Images
When dealing with complex images in your document, providing lengthy descriptions solely through alt text may not be the most effective approach. Alt text should ideally be concise, limiting its length to about 150 characters. Screen readers treat alt text as a single chunk of text, making it difficult for users to navigate or interact with lengthy descriptions.
Option 1: Add a long description
An easy way to provide long description for a complex graphic is to write the description in the text of the document. There is no character limit, you can add bullet points, tables, etc.
Option 2: Add a link to a footnote or endnote
If the long description interrupts the flow of a document, you can make a link to a footnote or endnote with a long description.
Option 3: Add a link to an External Resource
Sometimes, it helps to add a link to a website with a long description. However, this is less convenient for the reader. They have have to exit the document and open a web browser. It is also risky because you may forget about the web document and delete it or move it. This method does help though.