I think the point is to have the copyright and CC-licence note in such place so that a viewer a) wouldn't mistake the image as yours, and b) would find the author if they look for it.
So just ask yourself:
a) Are there places that might make someone think you've authored the images?
b) Where would someone go look to see who created the work?
Put the notice in those places. Usually this would just be the "about" section of the site. But there are many cases where a site contains some content that's clearly attributed to a different author than the rest of the site (e.g. a news article). If the image is used in such context, then a small caption near the image or a note near the content author's name is the way to go. Alt-text to the image is also a good option in these cases, but I don't think I would rely on it myself, because that sort of depends on whether the target audience understands that the image might have a hover text - not all audiences do.
Note: One possible source of mistaken authorship would be those silly "(c) Someone 2011" or such notices at the bottom of every page. So if you happen to have one of those, well, either just remove it, or add a notice like "except some CC-licenced images" there that then links to the page where you have the copyright information.
In any case, there's no need to contact the author for clarification. The whole point of CC is to make fair sharing possible without contacting the author. When the author placed their work under CC, they directly made the choice for others to be able to use their image under the terms that CC defines - including how they are attributed. That said, the author might of course be delighted to know that you've decided to use their image, so no harm in it either if you want to. :)