Accessibility & Search Engine Optimization
Adding image descriptions helps those who are visually impaired, or have certain sensory processing and/or learning disabilities, to view your site. Screen readers can read the image description, translating text-to-speech to describe the image.
If an image is unable to be displayed, the image description may be shown in its place (if a browser has been set to block images, for example).
Image descriptions help search engine crawlers to index your images properly, which will make it easier for people to find your website when performing a search related to your content.
Tips for writing 'good' image descriptions
'Image Description' is Format's terminology for 'Alt Text'.
Don't start with 'image of…' or 'picture of…'
Write in natural sentences. Imagine someone reading it back to you (with your eyes shut), would this create a picture in your mind?
Don't stuff your image descriptions with keywords and lots of commas, search engines can detect this and it won't improve your SEO
A few words will be enough, be concise
Adding incorrect descriptions can be worse than leaving them blank
Image descriptions are limited to 255 characters
Images and examples of good Image Descriptions
Image Description: “A metal sign in the desert that reads ‘viewpoint trail end’, in front of a large rock formation”
Image Description: “A latte in a white cup with a saucer, a silver spoon rests on the saucer”
Image Description: “A group of cows standing in a field, farmfields rolling into the distance behind them”
Images above from unsplash.com by photographers tyler-b, jason-wong and jakob-cotton respectively.
How to add Image Descriptions on Custom Pages
Tap the image and a mini-toolbar will appear above
Tap 'Image Description'
Thumbnail of the image to describe
Add your image description by describing the image, (an example is provided in a light color)
'Save Changes' for the whole page before leaving
For this image the description could be 'Woman wearing red dress seated on fallen log, sea and grey sky behind'.