This is going to sound crazy...but I've found Youtube to be a great source of low-res, relevant news thumbnails. Run your keywords through their video search,

This is going to sound crazy...but I've found Youtube to be a great source of low-res, relevant news thumbnails. Run your keywords through their video search, pluck out the thumbnails for the top search results. Remember, the AP and just about every other news organization is posting their footage to the site.