Should I copyright my artwork before showing it online?

"Visually Perceptible Copies
The notice for visually perceptible copies should contain all
three elements described below. They should appear together
or in close proximity on the copies.
1 The symbol © (letter C in a circle); the word “Copyright”;
or the abbreviation “Copr.”
2 The year of first publication. If the work is a derivative
work or a compilation incorporating previously published
material, the year date of first publication of the derivative
work or compilation is sufficient. Examples of derivative
works are translations or dramatizations; an example of
a compilation is an anthology. The year may be omitted when a pictorial, graphic, or sculptural work, with
accompanying textual matter, if any, is reproduced in or
on greeting cards, postcards, stationery, jewelry, dolls, toys,
or useful articles.
3 The name of the copyright owner, an abbreviation by
which the name can be recognized, or a generally known
alternative designation of owner.
1
Example © 2007 Jane
Doe
The “C in a circle” notice is used only on “visually perceptible” copies."

Then send yourself a copy of your artwork to yourself in the mail. Don't open the envelope.