Anytime you are adding new photos on your website, you can add Alt text on them.
Go to > Website > Pictures:
Click on the photo on which you want to add Alt text. From here you can add Alt text, we recommend you to add some keywords which describe what is on the photo. It doesn't have to be a full phrase, it can be several keywords only:
You can also add SEO to the photos of your accommodations:
Add your Alt text in all the languages activated for your website, and click on save.