The alt attribute, or title, these are not appropriate ways to give credit.
For that, you need to use phases like: “Permission granted by”, “Original work of”, “Photo use with permission of”. alt is to cover the use cases, title is somewhat non-important.
img element’s attributes alone don’t cover the type of license and use agreement.
Also, when using a specific photo, click on the author’s profile. It may have requirements of its own, such as to see how you use his/her work.
The best you can do: for every photo you use, contact the author. Ask for permission, ask for the manner he/she wishes the credit be given, and special requests he/she may have: a back link to his/her site, use it only for making the grass grow greener, etcetera. That’s how you deal with using other people work, unless it’s public domain.
And Wiki Commons Terms of Use say pretty clear:
http://wikimediafoundation.org/wiki/Terms_of_Use
As an author, you agree to be attributed in any of the following fashions: a) through a hyperlink (where possible) or URL to the article or articles you contributed to, b) through a hyperlink (where possible) or URL to an alternative, stable online copy which is freely accessible, which conforms with the license, and which provides credit to the authors in a manner equivalent to the credit given on this website, or c) through a list of all authors. (Any list of authors may be filtered to exclude very small or irrelevant contributions.)
But alt is only available in certain circumstances, and so is title. They don’t complement each other. Since neither offer a valid way to always give credit, none is suited.
