InsideStory Flashcards pegs their site as "the world's most interesting way to learn words." It is indeed a nice repository of free printable flash cards with interesting images that illustrate vocabulary words. About 1000 of these are available to us free through the site, and for a reasonable cost, the site sells boxed sets of vocab flashcards with "Cats & Dogs" and "Animals" themes for human (well, and certainly animal) vocabulary such as malodorous.
In terms of vocabulary, I am actually a big believer in the Isabel Beck school of thought that kids need Tier 2 Vocab (see all the words on the above page as examples) taught in context and repeatedly. As a result, I think given our service delivery models that it's really tough for vocab to be our sole responsibility, and it is best targeted in consultation with teachers and paraprofessionals. So really, I think that InsideStory Flashcards is more a great model of what you can do using images on the web than a resource in itself. For example, look at how the site approaches the creative and contextual selection of an image for the word frolic, above (the rolling-around polar bear). One could take any vocab word being targeted in the classroom and either pre-select or, even better if you can, have students pick and save images (click here to see how) for their vocabulary words (as this would involve the higher level skill of evaluation).
Think of trying a search like this for a vocab word like gregarious. See that SafeSearch Dropdown up there in the upper right? Make sure if you are using Google Images with students to set SafeSearch to "Strict" in order to avoid any embarrassing moments.