Alt text is the short description attached to an image so that someone using a screen reader can follow the page. It also shows up when an image fails to load. Most sites treat it as a box to tick. That’s a waste.
Writing good alt text means deciding what the image is doing on the page. That’s an editorial call, the same as picking a headline.
Describe the point
Take a photo of a port at dusk. If the post is about a shipping strike, the alt text should mention the idle cranes.
This is a container terminal on the north bank of the Tagus in Lisbon, shot in warm, low-angle light under a clear, pale blue sky. It looks like early morning or late afternoon.
The main subjects are two big ship-to-shore gantry cranes with red-and-white striped booms raised skyward. Their cabs carry YILPORT Liscont branding, and a third grey crane stands further right. Rows of red rail-mounted yard gantries fill the left edge and the right middle. Behind them, the red towers, suspension cables and truss deck of the 25 de Abril Bridge cut across the frame. The cranes and the bridge share almost the same shade of red, which gives the picture its strong colour unity.
Under the cranes, the container stacks show Maersk, Hapag-Lloyd and Hamburg Süd boxes in reds, oranges and blues. On the right is a long grey quayside warehouse with a “TS” logo. Across the river, the dry, scrubby hills of the south bank sit in a hazy blue-green band, with a few low buildings and what look like faint wind turbines on the ridge.
The foreground is calm blue water in a marina or basin. A blue-hulled work boat or tug sits at left, and a wooden-masted sailing ship stands at centre with a cluster of masts and rigging crossing the lower frame. Small sailboats fill the bottom right. The old and the industrial sit right next to each other, which is probably the picture’s best angle for a post.
If the post is about sunset photography, it should mention the color of the sky. Same picture, different alt text.
The sun is a white disc just above a dead-straight horizon, with an orange glow spreading out around it and a short glitter path on the water below. To the left, three stepped wooden pergolas stand black against the light, and people sit on benches under and beside them. A thin bright line runs from the top of the frame through the sun and out toward the lower right corner. The paving in front is almost black.
Skip “image of” and “photo of”. Screen readers already announce that it’s an image. Keep it to a sentence or two, and don’t stuff it with keywords. Search engines can tell.
When to leave it empty
Purely decorative images, like a divider line, should have an empty alt attribute so screen readers skip them. Leaving out the attribute entirely is different, and worse, because some readers will then read out the file name.
If you run a gallery with thousands of images, write alt text as you upload. Going back later never happens.