I'm running a tube site that has a thumbnail for each post/video (running via Wordpress, 500k posts). When I first created the thumbnails they were a fairly large size, around 640px in width each and because of that I was seeing a lot of traffic from Google Images.

After streamlining the site and resizing all of the thumbnails down to 170px rather than scaling them down I'm worried that Google isn't going to rank the images as high as they would be at a larger resolution, so is there a way to include the higher res versions and serve them to be indexed instead of the smaller ones?