The answer for the PDF portion of your question may be that there is simply
no corresponding attribute in the PDF format for graphics.
As for why you don't see them when you hover over the graphic, I don't know,
but just because you don't see them doesn't mean that a blind person using
special access software wouldn't hear them.
If you need this question answered definitatively, and you want to send me a
sample document, I'll be able to tell you whether I hear anything like an
alt tag when I move the cursor over a graphic.
hth,
Chip
