<p>I want to use <a href="http://alistapart.com/articles/sprites" rel="noreferrer">CSS sprites</a> on a web site instead of separate image files, for a large collection of small icons that are all the same size. How can I concatenate (tile) them into one big image using <a href="http://www.imagemagick.org/" rel="noreferrer">ImageMagick</a>?</p>

From the page you linked,
'montage'
is the tool you want. It'll take a bunch of images and concatenate/tile them into a single output. Here's an example image I've made before using the tool:
(source:
davr.org
)