We propose a web-based system for organizing the creation and the interaction in multimedia web-based environments that permit the collaboration among artists, audience, curators and publishers. This system uses a threelevel approach for handling security issues in multimedia projects. Especially, it applies authorization, digital certification and watermarking mechanisms on multimedia content. Authorization mechanism controls users operations in multimedia files based on a set of actions imposed on reallife scenarios. Digital certification mechanism ensures the secure content transfers among users and multimedia content server through Web. Digital watermarking mechanism handles copyright-protection and authentication issues that appear in multimedia systems applying a semantically secure watermark. The strength of our system considers multimedia content not as an atomic digital entity, but as a part of a project with a specific workflow that applies watermarking techniques in all its parts.