When you place a downloaded asset on your site, it is not always obvious what belongs in the alt attribute. Pasting in the description from the stock page is not necessarily the best answer. Here is what to write, and where not to write anything.
Write what the image is conveying
Alt text is read in place of the image where the image does not appear, or where the page is being read aloud. What belongs there is not what the picture looks like but what it is conveying in that spot.
"A woman in a blue top smiling" is less use in most contexts than "support staff answering an enquiry". As the next section says, though, there are cases where the right answer is to write nothing at all.
Leave decorative images empty
For an image that conveys nothing — the picture dropped in to break up an article — write alt="". Screen readers skip it. The W3C's accessibility guidance (WAI) states plainly that decorative images take an empty alt.
That is not the same as leaving the attribute off. Omit it and some environments read out the file name instead. Writing it empty is the statement that no description was needed.
Do not repeat the surrounding text
If a heading or caption right next to the image already says the same thing, alt can be empty. Otherwise the same sentence is simply read twice. The W3C guidance treats this case the same way.
Do not reuse the stock site's description
The description on a stock page is written for people searching for assets. It often includes the drawing style or the composition. Pasted straight in, that becomes noise when read aloud. Rewriting it for the context you put it in is the point.
How long it should be
One sentence is usually enough. For a diagram that needs real explanation, put the explanation in the body text and let the alt point to it. Long alt text is a burden in environments where it cannot be interrupted.
It is not there for search
You will see alt described as a place to pack keywords. That misses what it is for. Well-written alt text may help search as a by-product, but reversing the order costs the people using a screen reader.
About Visual Hub assets
Our asset pages carry descriptions in Japanese, English, Chinese and Korean, but those exist to help you find assets. When you place one on your site, rewrite it for the context. If it is there as decoration, an empty alt is the right answer.