You can easily add an image description (or ALT text) to an image in your postings. The text will be displayed when the image is hovered with the mouse cursor, and is used by screen readers.
Say, you want to embed the image.jpg from example.com. The BBCode to add the image and provide an ALT text for it would be:
[img=http://example.com/image.jpg]This is the image description text.[/img]