Hello all,
I have a second suggestion for the alt problem that will require the use of WAI-ARIA, namely aria-describedby. Drawback: That is currently only valid in HTML 5. But it would be the better solution since alternative text and caption text are two different things (see also http://en.wikipedia.org/wiki/Wikipedia:Alternative_text_for_images).
Using WAI-ARIA the solution would be:
1. If [[File:...|alt=...|...]] is given, use this text for the alt attribute. 2. If no alt text exists, use the “simplified” file name. 3. Add aria-describedby="ImageCaption" to the img tag. (With ImageCaption being the id of the caption.)
The screen reader then can associate the caption with the image.
Cheers, Maria
P.S. Please not that the handling of alt text currently differs depending on whether you use thumb, frameless or pixel (see http://de.wikipedia.org/wiki/Benutzer:Raymond/alt). It must be consistent.