This is an image with a logotype on the left saying W3C, and a text saying XHTML v1.1 on the right. The meaning of this picture is that these pages have been validated through an validation-service which controls the format of the pages to be valid XHTML. The picture links to the homepage where the validation-service can be found.

If no image shows up, you likely have a browser unable to view images in PNG-format.