When does an image need more than an alt attribute?

When a reader who cannot see it would still be missing something after hearing the alt text. A photograph of a beach rarely qualifies. A line chart with four series over twelve years always does, because the alt attribute can say what the chart is and cannot say what it shows. The test is not complexity for its own sake — it is whether the page's argument depends on detail inside the image.

Where does the long description go?

On the page, in text, near the image, and reachable by everybody. A details element under the figure, a paragraph in the body, or a link to an anchor further down all work. What does not work any more is the longdesc attribute, which was dropped in HTML5 and was never implemented consistently enough to rely on when it existed.

How should the paragraph be ordered?

Whole first, then parts. Someone listening cannot glance at the shape and then go looking for a number; they get the description in the order you wrote it and cannot scan back. So the first sentence says what the thing is and what it is for, and everything after that fills it in. The checker marks a description that opens on detail, because that is the one structural fault a long description can have.

Should the alt attribute still say something?

Yes, and it should be short. Give the image a name — what kind of thing it is and what it is about — and let the long description carry the content. An empty alt on an image with a long description leaves the image unnamed in the list of things on the page, which is exactly the wrong way round.

Long description checker for charts and diagrams

What it does

Some images are arguments. A chart with four series, a process diagram with a loop in it, a map with six labelled districts, a scanned form with a structure: the page depends on what is inside them, and no single attribute is going to carry that. The answer is a second piece of text — longer, in the body, available to everyone — and this route is where it gets written and read back.

The ruleset changes shape here. There is no character ceiling worth enforcing, so the length checks turn into structure checks: does the first sentence say what the whole thing is, is any single sentence so long that a listener loses the thread, is the paragraph one sentence pretending to be several. Those are the faults that make a long description hard to follow, and they are invisible on a screen where a reader can scan.

The order of operations

Name the thing

A line chart of rainfall. A floor plan. An organisation chart. One clause, and it is the clause the alt attribute should also carry, because it is what lets someone decide whether to listen to the rest.

Say what it is for

What the page is using it to show. A chart that exists to demonstrate one crossing point should say so before it reads out twelve years of values, because after that sentence the listener knows which number to wait for.

Then the parts, in a stated order

Left to right, largest to smallest, first to last — any order will do as long as you name it and keep to it. A listener who has been told the order can hold their place in it. A listener who has not is being read a list.

What else a file can tell you

A long description answers what the image shows. There is a second set of questions it never touches: when the photograph was taken, on what, at what exposure, whether there is a location in it, whether the caption and credit fields were ever filled in, and whether the file carries a provenance record saying where it came from. None of that is in the picture. All of it can be in the file.

That is a different tool and it is a good one: imagelookup.com reads the metadata blocks — capture data, IPTC caption and credit fields, GPS, lens, colour profile, provenance — and reports what is actually in them. It is worth running before you write, because a picture desk that filled in the IPTC caption has already written half of what you were about to. This page does not read any of that, and the omission is deliberate rather than a gap: the two jobs want different instruments.

Two things this route will not do

It will not read the numbers off your chart. Pulling values out of an image is text recognition dressed up as description, and it is refused here in product rather than merely left unbuilt. A chart that is mostly axis labels and tick text is caught by the same edge-density test that catches screenshots and is declined with the arithmetic shown.
It will not know which order matters. The structural check can see that your first sentence went straight to detail; it cannot see that you described a flowchart from the bottom up, or that the important series is the third one. Those are judgements about what the image is for, and the page it sits on is the only thing that knows.

Questions people ask

When does an image need more than an alt attribute?: When a reader who cannot see it would still be missing something after hearing the alt text. A photograph of a beach rarely qualifies. A line chart with four series over twelve years always does, because the alt attribute can say what the chart is and cannot say what it shows. The test is not complexity for its own sake — it is whether the page's argument depends on detail inside the image.
Where does the long description go?: On the page, in text, near the image, and reachable by everybody. A details element under the figure, a paragraph in the body, or a link to an anchor further down all work. What does not work any more is the longdesc attribute, which was dropped in HTML5 and was never implemented consistently enough to rely on when it existed.
How should the paragraph be ordered?: Whole first, then parts. Someone listening cannot glance at the shape and then go looking for a number; they get the description in the order you wrote it and cannot scan back. So the first sentence says what the thing is and what it is for, and everything after that fills it in. The checker marks a description that opens on detail, because that is the one structural fault a long description can have.
Should the alt attribute still say something?: Yes, and it should be short. Give the image a name — what kind of thing it is and what it is about — and let the long description carry the content. An empty alt on an image with a long description leaves the image unnamed in the list of things on the page, which is exactly the wrong way round.

Write a long description for a complex image