Alt text is a description of an image that can be read out-loud by accessibility tools. Go to Settings > Accessibility and check "Require alt text before posting." That will force you to enter alt text on your images. It's also a good opportunity to "add color 😉" to your post.

Alt text is offered when you post a photo. It is descriptive text for the visually impaired using screen readers.

accessibility.huit.harvard.edu/describe-con...

To add alt text to a picture 1. If the pic has text, copy it first. 2. Add picture. 3. Press the +ALT button on upper left corner of photo. 4. Paste the text you copied, or describe the pic, 2k characters, hit save 5. Finish and post. People can click the ALT to get the summary of pic