Notification is not required for protection.
It depends on when the work was created. For works created before 1989, it may be required; the courts have not ruled as to whether a copyright notice is required on works first created before the U.S. became part of the Berne Union. For works created after 1988 the answer is simple. No. A copyright symbol has not been necessary in the US since 1989. There are advantages to putting the copyright symbol or copyright notice on a work even if not required, including eliminating the "innocent infringer" defense.
It is possible that Charter has sent you a notice of copyright infringement.
I have not received a Spectrum copyright infringement notice.
The copyright notice and other cataloging data is usually on the reverse of the title page.
Same as copyright notice for anything else: Copyright, or copr. or circle-C, the date of copyright and the author or other copyright owner's name. E.g., "Copyright 2012 Walt Disney Productions, Inc."
A notice can be added as soon as the project is complete, even before registration.
I have not received a notice of copyright infringement related to the use of spectrum.
It can be. Notification is not required for protection.
Indeed. The copyright symbol, ©, indicates the work is (or was) protected by copyright; such notification is not required for protection on publications after 1989 in the USA. Many works published prior to 1963 had © or "copyright" notice, but expired after 28 years.
There is no way to share your works without some risk of infringement. Your best option is include a copyright notice in the image itself, as well as embedded in the metadata of the digital file.
In the USA and more than 160 other countries of the Berne Union, the answer is that no copyright notice is necessary. This has been the law in the USA since 1989, meaning that things published prior to that, by US authors in the USA, were required to have copyright "notice", whether using the word "copyright", "copr." or the copyright symbol, followed by the publication date, followed by the name of the copyright owners. Laws for Copyright notice in other countries may vary.
If you have received a notice of copyright infringement from Spectrum, it means they believe you have violated copyright laws by using or sharing protected material without permission.