Posting a design on your website and copyright law

[bakdrafy]

Does posting a design on your website then give you a copyright on that design? ...say e.g. if somebody takes it and slightly alters it and uses it on their site?

[adjusterjack]

Does posting a design on your website then give you a copyright on that design?

Yes.

In fact, you have copyright protection at the moment of its creation. But registration gives you even more protection.

You can study up on copyright at:

U.S. Copyright Office | U.S. Copyright Office

Depending on what the design is, you may, instead, have trademark protection.

https://www.uspto.gov/

e.g. if somebody takes it and slightly alters it and uses it on their site?

Very likely infringement for which you may seek compensation.