I once saw a catalog of pictures of enzymes which showed them to have
a striking, crystalline structure.  Can you find me some of these
pictures--especially for those enzymes which have to do with
biological function--plus explanations for why this is the case (i.e.,
the reason for the structure)?

The following page has some small pictures of enzymes in a crystalline
state:
http://www.hamptonresearch.com/stuff/gallery.html

I could try to explain part of the reason why these crystals form, but
it has been a few years since my last chemistry course and I don't
want to give you any wrong information. In short, the enzymes form
crystals because of the nature of their chemical bonding to one
another to form what are known as lattice structures. The amazing
regularity of the pyramidal, cubic, and other shapes of the crystals
is due to the way these lattices form for different enzymes (which are
proteins, by the way). The actual reasoning behind lattice formation
is quite complex (at least for me) since I barely learned about it in
first-year university chemistry. Perhaps a researcher who is a
chemistry guru can provide a simplistic explanation.

There are plenty of pictures of enzyme crystals on the web;
try writing "enzyme crystal structure" in the google Image searching page.

But the crystal structure usually has no connection to its biological function.
The crystals are grown in the laboratory so that the scientists can 
solve the
molecular structure of the enzymes using X-ray crystallography techniques.

In other words, the crystals are only a device used in order to
get the structure of the enzyme molecule. The knowledge of the structure is used
to understand the function of the enzyme, in order to design drugs to act
with or against the enzyme, etc etc,

In the living cell, the enzymes exist as individual molecules, not in in
the crystalline form.

P.S. I'm a research scientist who studies protein structures