Facebook allows you to add alternative text (alt text) to images you upload from a desktop browser. Currently, adding alt text to images is not available from the Facebook mobile app.
Select Photo/Video from the News Feed or timeline.
Choose the photo you want to upload to Facebook.
Select Edit Photo. You may need to hover over the photo with cursor or tab to the photo options using tab key.
From the list of editing options, select "Alt Text."
Select Override Generated Alt Text.
Enter a descriptive alt text in the edit box.
Note: Facebook has a 100 character limit on alt text.
Choose Save button.