A key issue in visual languages is the specification of the relationship between pictures and their meaning. To do this, we introduce a class of declarative, constraint-based picture specification languages. These extend previous approaches to picture specification based on constraints and grammar formalisms. As the languages are defined as instances of an abstract language scheme, we ensure they share a common declarative semantics and operational semantics. The declarative semantics allows both people and machines easily to reason about a specification independently of any underlying implementation. The operational semantics permit both the generation and recognition of pictures defined by the specification.