For web there is a standard for alt-text in HTML that screen readers and search engines can use. You can add this in Studio Pro when you open the settings for the image
For native mobile, I don't think there is a standard yet? So if you mean for your native app to be usable with a screenreader for accessibility, you could probably display a string attribute on the same page as the image, or add logic to display it at certain settings.
see also this.