...
Import your image into InDesign
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?
...