In this paper we present the construction of an ontology that represents the structure of the MPEG-7 visual part. The goal of this ontology is to enable machines to generate and understand visual descriptions which can be used for multimedia reasoning. Within the specification, MPEG-7 definitions (description schemes and descriptors) are expressed in XML Schema. Although XML Schema provides the syntactic, structural, cardinality and datatyping constraints required by MPEG-7, it does not provide the semantic interoperability required to make MPEG-7 visual descriptors accessible by other domains. The knowledge representation provided by the ontology can be used to develop tools which perform knowledge-based reasoning. For the construction of the ontology we use the RDFS ontology language. We present the problems that occurred, mainly, due to the RDFS modelling limitations. Finally, we propose a way to apply reasoning using the VD ontology.