Copyright law says that every work done after 1989 is automatically considered copyrighted. They do not need to announce the copyright. You can't steal others works unless they put it out there as free to use. If she/he/they say remove it, you must.

NOTICE OF COPYRIGHT
The use of a copyright notice is no longer required under U.S. law, although it is often beneficial. Because prior law did contain such a requirement, however, the use of notice is still relevant to the copyright status of older works.

The art used if copyrighted is credited and used with permission. Other works of art are believed to be public domain. If you find your copyrighted art herein, I will remove it or credit you as you so desire.

Background on the following page(s): What's New