mezzoblue revised image replacement:
"Plenty of new and interesting revisions to the original Fahrner Image Replacement technique have sprouted up recently. This is an attempt to consolidate them, so that perhaps we can decide on the official replacement.

Requirements: the replacement must solve the screen reader problem, and it must address the 'images off, css on' problem. It is also hoped that a solution will be found that reduces the need for empty elements. The successful technique must work in browsers back to 5.x, but as of the time of writing none of these appear to fail so browser support matrices will be spared.

The two most promising techniques, Phrak and Gilder/Levin are available on a reduced page for screenreader testing."

http://www.mezzoblue.com/tests/revised-image-replacement/