No parece tan difícil entenderlo:
http://www.w3schools.com/tags/tag_img.asp Cita: Atributo: crossorigin
Posibles valores: anonymous, use-credentials
Descripción: Allow images from third-party sites that allow cross-origin access to be used with canvas (Básicamente: Permite acceso a imágenes de sitios de terceros para ser usadas en <canvas />)
Hay una explicación más amplia en:
https://developer.mozilla.org/en-US/...FElement%2FImg Cita: crossorigin
This enumerated attribute indicates if the fetching of the related image must be done using CORS or not. CORS-enabled images can be reused in the <canvas> element without being tainted. The allowed values are:
anonymous
A cross-origin request (i.e. with Origin: HTTP header) is performed. But no credential is sent (i.e. no cookie, no X.509 certificate and no HTTP Basic authentication is sent). If the server does not give credentials to the origin site (by not setting the Access-Control-Allow-Origin: HTTP header), the image will be tainted and its usage restricted..
use-credentials
A cross-origin request (i.e. with Origin: HTTP header) is performed with credential is sent (i.e. a cookie, a certificate and HTTP Basic authentication is performed). If the server does not give credentials to the origin site (through Access-Control-Allow-Credentials: HTTP header), the image will be tainted and its usage restricted.
When not present, the resource is fetched without a CORS request (i.e. without sending the Origin: HTTP header), preventing its non-tainted used in <canvas> elements. If invalid, it is handled as if the enumerated keyword anonymous was used. See CORS settings attributes for additional information.
Si no te es suficiente esa información, entonces investiga sobre peticiones CORS (CORS requests)