I'd suggest contacting the photographer to see what their "specified manner" might be, as a matter of courtesy. But the actual legal requirements are (by design) quite reasonable for reuse.
If you read the actual license terms in their full, legal-language form, the key relevant point appears to be:
Such credit may be implemented in any reasonable manner; provided, however, that in the case of a Derivative Work or Collective Work, at a minimum such credit will appear where any other comparable authorship credit appears and in a manner at least as prominent as such other comparable authorship credit.
In other words, you can put the attribution with your other copyright notices and about-this-site stuff. You don't have to put it as a caption directly with the image at all. (Although doing so would probably be nice.)
The Creative Commons wiki has a "Marking/Users" page, which specifically suggests that images used online should be put in a "credit list".
The following is from the Creative Commons FAQ:
How do I properly attribute a Creative Commons licensed work?
All current CC licenses require that you attribute the original author(s). If the copyright holder has not specified any particular way to attribute them, this does not mean that you do not have to give attribution. It simply means that you will have to give attribution to the best of your ability with the information you do have. Generally speaking, this implies five things:
If the work itself contains any copyright notices placed there by the copyright holder, you must leave those notices intact, or reproduce them in a way that is reasonable to the medium in which you are re-publishing the work.
Cite the author's name, screen name, user identification, etc. If you are publishing on the Internet, it is nice to link that name to the person's profile page, if such a page exists.
Cite the work's title or name, if such a thing exists. If you are publishing on the Internet, it is nice to link the name or title directly to the original work.
Cite the specific CC license the work is under. If you are publishing on the Internet, it is nice if the license citation links to the license on the CC website.
If you are making a derivative work or adaptation, in addition to the above, you need to identify that your work is a derivative work i.e., “This is a Finnish translation of the [original work] by [author].” or “Screenplay based on [original work] by [author].”
In the case where a copyright holder does choose to specify the manner of attribution, in addition to the requirement of leaving intact existing copyright notices, they are only able to require certain things. Namely:
They may require that you attribute the work to a certain name, pseudonym or even an organization of some sort.
They may require you to associate/provide a certain URL (web address) for the work.
If you are interested to see what an actual license ("legalcode") has to say about attribution, you can use the CC Attribution 3.0 Unported license as an example. Please note that this is only an example, and you should always read the appropriate section of the specific license in question ... usually, but perhaps not always, section 4(b) or 4(c):
In an awesome display of irony, the above text is marked as licensed under the Creative Commons Attribution 3.0 License, but the page does not provide clear and concise way to provide attribution for itself. It appears to be a collaborative work by these authors.