In more detail
In HTML, an image is written as <img src="…" alt="…">. The alt attribute was designed for accessibility. It's what assistive technology reads in place of the image, and what browsers display if the image fails to load. The W3C's Web Content Accessibility Guidelines require text alternatives for meaningful images.
Google's image SEO best practices say it uses alt text, along with computer vision and the page content, to understand images. Google recommends alt text that is "useful, information-rich" and uses keywords appropriately, and warns against filling alt attributes with keywords, which it calls keyword stuffing.
Good alt text describes what matters about the image in context. A product photo might say "Navy wool overcoat, single-breasted, shown from the front". A decorative divider needs an empty alt attribute (alt="") so screen readers skip it. If a linked image has no text around it, its alt text acts as the link's anchor text.
Why it matters when hiring an agency
Alt text is a small, routine task, but how an agency handles it is revealing. Good agencies treat it as accessibility first and SEO second. They write descriptions for meaningful images, leave decorative images empty, and avoid stuffing. If an audit flags "missing alt text" across thousands of images, ask whether those images are meaningful or decorative before paying for a mass rewrite.
For ecommerce, recipe, real estate and travel sites, where image search can drive real traffic, alt text and image file names deserve more attention. Ask the agency how it will prioritise.