Okay, IŽll keep this short, as their is really large range of sites online. So just a few specific things !
http://www.afriqueindex.com/A tremendous list of sites. Look in the section "Arts/Culture". French language, but still useful for those who do not speak this language.
http://www.africa-photo.comAfrica-photo-com is the most comprehensive Africa photo databank in the internet.
At the present time you can search in > 11.000 photos from 1870 to the present time