Another place to ask in the Standards & Accessibility forum: http://forums.adobe.com/community/acrobat/accessibility_%26_standards
I don't have Word 2010, but in Word 2007 you specify the Alt text for an image by right-clicking the image, select Size (of all things!), and there is an "Alt Text" tab where you can enter the text you want. Then convert to PDF with PDFMaker, as opposed to printing to the PDF printer, so that it's tagged. There's much more to creating an accessible document, and the folks over in the other forum are helpful.
Thank you, George! I think I've found what I was looking for at the other forum - they list some free tutorials!