Visual Dictionary by Webster is such a great resource for schools and in time could end up being the ultimate online dictionary. My only reason I say "in time" is because it is still relatively new and there needs to be more terms/words added before I would use it consistently. For example, I did a search on a bunch of technological terms, and until I made it very vague say w/ the word technology or computer, nothing came up. Although, what VD does have is quite robust; 20,000 terms w/ contextual definitions and 6,000 images.
Visual Dictionary has such an abundance of options for the teacher/student this is a definite resource that every educator should check out. For instance, there is a game section which is great for labeling pictures and so on. Also, there is a School Lesson Plan section on a variety of subjects for ages 4-12 and Highschool. For me personally, my favorite section is the "Tools" section which lets a blogger/web designer grab the code and use any image in their site.
I highly recommend giving Visual Dictionary a look, you'll be happy you did!!!
For more info click here.
BTW, I did test out the filter/content aspect of VD and noticed that you can't pull up any inappropriate images/terms (even bad medical terms). This is great for schools who need to abide by CIPA/COPPA compliance. Although, I would need to see a strict policy in place before I let students start using this un-assisted.