2012
HTML5: Techniques for providing useful text alternatives: developer edition
This document contains author conformance requirements for use of the alt attribute in HTML5 and best practice guidance for authors of HTML documents on providing text alternatives for images.
This specification is an extension to the HTML5 specification [HTML5]. All normative content in the HTML5 specification, unless specifically overridden by this specification, is intended to be the basis for this specification.
TinyPNG – Compress PNG images while preserving alpha transparency
by 5 othersAdvanced lossy compression for PNG images that preserves full alpha transparency.
2011
Make your images accessible | Yahoo! Accessibility Library
Images make web pages interesting. So let’s make them useful for everyone, not just those that can see them.
Five kinds of 'alt' text
What’s less commonly known is that there are five different classes of image used on web pages and each of those images requires a different approach to writing the ‘alt’ attribute.
Text alternatives for images: some examples « Writing for the Web
The same image may require a different text alternative when it plays a different role or is used in a different context.