According to many sets of accessibility regulations and guidelines, including the US Section 508, and the W3C’s WCAG 2.0, image elements on web pages must have text equivalents to be considered accessible. This is accomplished by specifying alt text for the image.
(curated by Lifekludger from 10 Guidelines for High-Quality Alt Text)
Complete article at source: http://www.katherinelynch.org/content/10-guidelines-high-quality-alt-text
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