I typed "C-t d" after marking files in Dired, or "M-x image-dired" followed by a directory name, to display thumbnails in a thumbnail buffer. After highlighting a thumbnail in the thumbnail buffer, I typed RET to display a sized version of highlighted image in another window (image-dired-display-image). The problem is that no matter what image is selected in the thumbnail buffer (either using the left/right arrow followed by RET, or using space/backspace, as instructed in the Emacs manual), the display-image window always displays the same image, which is the image opened first in this way in an Emacs session.
Upgrading Emacs from 24.4 to 24.5 solves the problem, although I am not sure what is wrong with my built of Emacs 24.4. (I compiled both versions of Emacs from source on Mac OS X Yosemite.)