Note: If you're not running a WordPress blog and using its built-in gallery feature, the following will be of no interest to you; it's posted here mainly to make it easier for me to find in the future, when I forget it once again.
WordPress includes a simple-but-usable gallery feature. Unfortunately, posts with embedded galleries display a thumbnail for every image in each gallery—and there are no options to limit the display of thumbnails. While fine for shorter galleries, such as this one, if you've got a lot of images, this can make for an ugly page.
What I wanted was the ability to include an image gallery in a post, but not show a thumbnail for every picture in the gallery. Ideally, I'd just be able to use the first image from the gallery, or perhaps even a text link. After a lot of fruitless searching, I finally found the very simple answer in a post by malissas in this thread.