This is a subject I've devoted a lot of time investigating. I am an avid
photographer with thousands of photos saved on my computer and on
various external devices. I came to linux from windows where I had found
the -perfect way of cataloguing and working on my photos. I am still
looking into a suitable solution with linux but have meanwhile come up
with this:

As with windows I think one needs two seperate programs. One for
cataloging and one for working. I think that in windows ACDsee does both
but I never really used it. In Linux for work I suggest using Gimp which
is as good as photoshop (and free :-) ).
There are other options, less elaborate, of course. My desktop is KDE so
I know their programs better than gnome ones, but there are equivalent
ones in gnome. For work one can also use Digikam (fspot in gnome). For
me the big problem was cataloging. Digikam an most other programs in
Linux that I looked at required the importing of all photos to a central
folder. Because of the fact that I have photos all over my computer (and
off the computer) that system doesn't suit me.
My solution for cataloging isn't final. I am looking into two programs
that seem to answer my specific needs, The one program is Kphotoalbum. I
am still trying to understand how to really use the program. I recently
discovered a program called imgseek which can be installed with "apt-get
install imgseek". This program seems to be the real answer I'm looking
for. It searches all hard drives and leaves the folders in place and
builds a catalog like windows with folders, subfolders and photos in a
database format. It doesn't import photos to one folder but builds
thumbnails. As I said this is still under investigation.