What if you can't see the image?
People with low vision, or no vision at all, can't see the images you put on your pages. We create alternative text, or "alt" text, to tell screen-reading software
to ignore a decorative picture completely when reading the page aloud, or
to convey the same information that the image does.
Decorative images usually don't need descriptive alt text.
They may exist on the page to make it look pretty.
They may be repeating information that is already on the page as text. In that case, adding alt text to the image is redundant.
Informative images convey some kind of information.
What happens if you remove it from the design?
If the reader is missing information because the image is gone, then it means the image is informative and needs alt text.
The "alt" tells the same tale that the image does.
A good "alt" is as much art as it is science. The article at https://axesslab.com/alt-texts/ provides an excellent overview of writing good alternative text.