Yasnippet trigger not working

Question

I have installed yasnippet, and confirmed that the minor mode is active, but I can't seem to trigger expansions.

This is in my init.el file:

(add-to-list 'load-path
             "~/.emacs.d/packages/yasnippet")
(require 'yasnippet)
(yas-global-mode 1)

(setq yas-snippet-dirs
      '("~/.emacs.d/snippets"                 ;; personal snippets
        "~/.emacs.d/snippets/yasnippet-snippets.el"
        ))

When I edit an org-mode file, for instance, and type img then TAB, nothing happens. I can't get any snippets to expand. Am I missing something?