Image SEO: Using Alt Text, File Size and Formats the Right Way

Images account for the largest share of the data volume on most websites. That is exactly why they often decide whether a page loads quickly or feels sluggish - and whether Google rates it as high quality. The good news: image SEO comes down to a handful of clear rules that you learn once and then apply consistently. We build websites for a fixed price and run seven of our own brands in production - the points below are exactly what we watch out for on every project ourselves.

Why image SEO is about more than just pretty photos

Image optimization brings together three things: load time, accessibility and discoverability. Fast images improve your Core Web Vitals, especially your LCP score (the largest visible element, which is often an image). Good alt text helps screen readers, and therefore people who cannot see - and, as a nice side effect, Google image search too. Sensible file names and formats round it all off. So you are never optimizing for the search engine alone, but always for real users as well. That is the honest core of it: when a page gets better for people, the rankings usually follow on their own.

Alt text: short, specific, descriptive

Alt text (alternative text) describes what can be seen in an image. It is read aloud when someone uses a screen reader, and shown when the image fails to load. Write it as if you were briefly explaining the image to someone over the phone.

• Describe the content, not the obvious: Not image or photo of, but the subject itself, straight away.
• Keep it short: A brief sentence is usually enough. Nobody needs an essay.
• Avoid keyword stuffing: A fitting search term is fine when it genuinely belongs to the image. A string of terms strung together does harm.
• Purely decorative images get an empty alt text (alt=""). That tells the screen reader to skip them - exactly right for dividers, patterns or ornamental graphics.

An example: instead of photo1 or nice-image-seo-marketing-agency, you write something like: Tradesperson inspecting a solar panel array on a house roof. That is honest, helpful and, as a bonus, search-engine friendly.

File size: the biggest opportunity to save

Most sites serve images that are far too large - often several megabytes for a photo that appears only a few hundred pixels wide on screen. This is where the biggest lever for speed lies.

• Serve the right resolution: An image that is displayed at a maximum width of 800 pixels does not need to be uploaded at 4,000 pixels. Scale it down before uploading.
• Compress deliberately: A quality level of around 75 to 85 percent is barely distinguishable from the original for photos, yet saves a huge amount of data.
• Target values as a rule of thumb: content images under ~150 KB, large hero images under ~300 KB where possible. These are guidelines, not dogma - but they help you judge by eye.
• Use responsive images: With the srcset attribute, a smartphone loads a smaller file than the desktop does. That way every device only gets what it actually needs.

Choosing the right format

The format determines the ratio of quality to file size. A simple guide:

• WebP: Today the default choice for almost all photos and graphics. Noticeably smaller than JPG at the same quality and supported by every current browser.
• AVIF: Smaller still than WebP, but a little slower to generate. Worth it when you want that last bit of speed and provide a fallback.
• JPG: A solid classic for photos when WebP does not fit for some reason.
• PNG: Only for images that need a transparent area or sharp edges (logos, screenshots). For photos, PNG is almost always too large.
• SVG: The best choice for logos, icons and simple graphics. Vector based, so razor sharp at any size and tiny in file size.

Technical details that often get forgotten

• Descriptive file names: solar-panel-roof-mount.webp says more than IMG_4821.webp - for Google too.
• Lazy loading: With loading="lazy", the browser only loads images once they come into view. The hero image right at the top, however, should not be lazy loaded, otherwise your LCP suffers.
• Fixed image dimensions: Specify width and height so the layout does not jump while loading. This noticeably improves your CLS score.
• Image sitemap: On very image-heavy sites it can help image search - on smaller sites it is usually not needed.

What you really need - and what you don't

Honestly: you do not have to optimize every image by hand. A good CMS or build process handles scaling, compression and format choice automatically - leaving only the alt text as a manual task for you. If your site is currently struggling with huge images, that is almost always the most rewarding first step toward better load times and a more stable ranking. AVIF, image sitemaps or sophisticated srcset setups are worthwhile extras, but not a must to get started. Begin with size and format - the rest will follow.