Dear Mark,
Sorry for the delay, I had to investigate a bit.
Short answer: there is not a way to do this right now.
We are putting in alt text, it generically says "Logo image". We have an option
for other images to describe them as "decorative" - this is something standard
that is done when an image is not really content, but is a decoration. It gets
an empty string for the alt-text.
Question: should we have a #shortdescription, or similar? Default to "Logo
image" if not provided? Or should we not solicit any alt-text and always mark
this image as a decoration? Or an exclusive-or: decorative or #shortdescription?
Rob