The United States Copyright Office has an online system for registering your copyright. While the system is the preferred way to register basic claims for . . . visual arts works,” an artist going trough the registration process would have to answer questions such as whether the work has been published. A question not easily answered by someone unfamiliar with the Office’s guidance on the matter.
Regarding publication, the Copyright Office says:
A work of art that exists in only one copy, such as a painting or a statue, is not regarded as published when the single existing copy is sold or offered for sale in the traditional way, such as through an art dealer, gallery, or auction house.
Once you complete a work, (assuming it hasn’t been published) you can photograph it, register the work, and upload those photographs directly through the eCO to complete the registration. Using the online system also save you up to $50 over completing the application in paper.
Registering and properly labeling your copyrighted work is an important step in protecting yourself from people illegally copying your work.