Give context to search engines and website visitors who cannot interpret images.
If your website has no CMS capabilities, you will need to consult with your developer or find one to assist you with the next steps.
Open the image settings inside your CMS and find the alt text field. In WordPress, for example, this will show in the image’s block settings.
100-125 characters are usually sufficient to describe an image comprehensively. There’s no need to include image of, or picture of, in your alt text.
If the opportunity is there, include the page’s target keyword in at least one of your images’ alt text. This allows search engines to find your keywords on the page, and understand which phrases the image should rank for in image search.