Whenever you upload an image, you can set alt text within the editor via the field provided in the media panel.
In many instances, you may set portrait and landscape alternates for images. Because these can differ in content, you can set different alt text for them too.
Please note that HTML video does not support alt text, so this option will not be available when uploading video. Instead, you may opt to use closed captioning, or the caption field.
Read more about accessibility in Shorthand here.