You can select the entire page, the entire visible area, or a custom area for the screenshot which can then be copied to the clipboard, saved as a local imaged file or can be published to WordPress as a new blog post.
This extension appears to be based on another useful extension, Screengrab, so the interface is pretty much the same.
Note that posting to WordPress requires the XML-RPC feature (WordPress 2.5+) enabled. Thanks Downloadsquad.
If you need to add annotations before uploading the image, also check out the review of DashBlog.