Photo: Bokeh / Tomedia. Ancient Stone Amphitheatre with Sunlight.
An image can help somebody understand what you are selling, show them the work, or explain something a paragraph struggles to describe. It can also be a very large file that slows down the page while contributing absolutely nothing to it.
Image optimisation gets reduced to filenames and alt text surprisingly often. Those details are useful, but I would start a step earlier. Why is the photograph on this page, and what is somebody supposed to learn from seeing it?
If you know that, most of the practical decisions become easier. You can choose the right crop, put it near the relevant explanation and describe it without trying to force a keyword into every available field.
Show the thing people need to see
An informational image should be readable at the size somebody will actually view it. A detailed screenshot shrunk until the text disappears may technically illustrate the subject while being impossible to use. A product photograph cropped away from the feature being discussed has the same problem.
Keep the image and its explanation together. A caption can identify the relevant detail or add context that the image alone doesn’t provide. If there is nothing useful to add, a caption doesn’t become compulsory just because the template has room for one.
A photograph of yellow flowers in a garden can have a filename and description that say exactly that. Adding “SEO image keyword” to the alt text doesn’t make the flowers any easier to understand.
I would apply that same discipline to more complicated material. Describe what the image contributes in this context. Don’t use the description as an extra advertising slot that somebody using assistive technology has to sit through.
The file has to travel to the reader
A photograph coming straight from a camera or an editing export may be much larger than the page needs. Putting it in a small box with CSS doesn’t necessarily make the downloaded file smaller.
Check the dimensions and the actual file weight. Serve an appropriate size, compress it without destroying the useful detail, and use responsive image options where the site supports them. The full-size version might belong in a gallery or download. It doesn’t need to accompany every small preview.
Google’s image guidance covers descriptive filenames, relevant surrounding text, alt text and responsive images. It also recommends standard HTML image elements so images can be discovered. These are implementation details worth getting right once in the site’s image component, rather than fixing the same avoidable problem on every new article.
Then look at the page. Does it reserve space for the image, or does the paragraph jump down when the photograph arrives? Does the main image appear promptly? Can you still read the detail on a phone? The file passing an upload check doesn’t answer any of those questions.
Alt text has a reader
Useful alt text depends on what the image is doing. An informative image needs an appropriate text alternative. A purely decorative one can have an empty alt attribute so it doesn’t interrupt the reading experience. Complex information may need a fuller explanation in the page itself. The W3C image guidance explains these different cases.
That is why automatically attaching the same keyword-heavy sentence to every image is such a poor solution. It ignores the image’s purpose and the person who needs the description.
You can use tools to help prepare the text, just as you can use them to resize the file. Somebody still needs to check that the description is accurate. If the software confidently names something that isn’t there, the fact that the field is populated has not improved the page.
Start with the important images on a few important pages. Check their size, position and description, then open the pages on a phone. The image and performance sections of The Big Book of SEO on Ephemera give you the other checks to work through.