Artist with Copyright Infringement Issue One-Time

Client is a visual artist who designed a Colorado state welcome sign (which is no longer at the state border). Client discovered that a business in Colorado and in Florida are using a photograph of this sign online for their commercial use. Upon further research, Client discovered that a few well-known stock image websites are selling or offering this image for free. Client has had the registered copyright on this artwork since 1991. Client is seeking legal assistance to figure out how to proceed and resolve this matter.