http://distrowatch.com/
It doesn't provide pics for the most part, but I belive
most of the distros available are listed there. It also give
links to the distros website where there are normally
screenshots and install instructions.
Or, the website that got me started,
http://www.linux.org/
They have a neat search filter that breaks the distros into
type (i.e. minimalist,debian based,etc) language preferance
and platform. Again, it sumarizes the distro and then
provides links to the distro homepage.
I'm sure that there are many more sites like these, but
I only know those 2 for sure.