Change “post thumbnail” to “post image”

I’m planning a theme that will use varying sizes of thumbnails. Actually just varying sizes of images, and one of them will be a thumbnail. Since the new thumbnail feature seems to work exactly like I want it to, is it possible to change what the feature says in the admin menu from “Thumbnail” to “Image”? I don’t need/want to change the functions itself, just the box and link titles that are displayed to the authors.