More experimentation on this front:
When created with Word 2007, the alt-text does not survive a DOCX to EPUB conversion. "Image" is used for all alt-text image properties regardless of the alt-text that's defined via Word 2007.
If I open that same docx with LibreOffice Writer and immediately save and close it (no changes), then the alt-text defined in the DOCX document will appear in the alt-text properties of the resulting EPUB.
So at the very least, DOCX documents created with Word 2007 will not have the alt-text properties of their images carried over to subsequent calibre conversions. I don't have a newer version of Word to test.
Not sure what that means--other than, "don't use Word 2007 DOCX as your source format if you want alt-text properties to carry over to your calibre conversions".
