For example, if an image of a magnifying glass is used for a search button, the alt-text would be "Search" rather than "Magnifying glass".
(an image of a magnifying glass; is used; for a search button) (the alt-text; would be; Search" rather than "Magnifying glass)
an image of a magnifying glass the alt-text
is used would be
for a search button
Search" rather than "Magnifying glass