Filling in the image alt text field when you upload a photo to your page is an important practice for ensuring a website is accessible to all visitors. But image alt text also improves your SEO by providing search engines with context about the media on your pages.

By writing a one-line description of every photo you upload to your page, you are telling search engines what your photo depicts. 

Google considers photos in general to be appealing content. But photos with alt text that is relevent to your page's keywords can be seen as SEO-enhancing content. However, you should not stuff your image alt text with keywords at the cost of your alt text being a clear and informative description of your image. An alt text description, even without keywords, will enrich a search engine's understanding of the content on your page.

There is a direct connection between accesibility and SEO as well: search engines do not want to offer links to pages that have content that is not accessible for all. Accessibility matters to SEO. Using image alt text is rewarded.

Image file names matter as well

Your image's file name also matters for SEO.

Uploading a photo to your page that is titled "IMG_0226" provides no context for Google to understand what your photo depicts.

However, a photo titled "Engineering Building at Memorial University" provides context, and if this photo is placed on a page for the Faculty of Engineering website, the image file also contains quality keywords.