If your app lets the user share images, consider implementing the possibility for them to add an alt text for the image, so it can be used as an accessibility label when consumed by other users. Twitter or Slack have nice flows for doing this.

The Twitter app is open. It shows how when adding an image in the tweet composing screen, there is a button to alt text that opens another screen that lets you write a description of up to 1000 characters. Then, if a tweet has an image with alt text, it shows an ALT badge in the bottom left corner that can be selected to open a bottom sheet with the image's alt text.