This paper presents the system architecture of a Content-Based Image Retrieval system implemented as a web service.The proposed solution is composed of two parts, a client run-ning a graphical user interface for query formulation and aserver where the search engine explores an image repository.The separation of the user interface and the search enginefollows a Service as a Software (SaaS) model, a type of cloudcomputing design where a single core system is online andavailable to authorized clients. The proposed architecturefollows the REST software architecture and HTTP proto-col for communications, two solutions that combined withmetadata coded in RDF, make the proposed system readyfor its integration in the semantic web. User queries areformulated by visual examples through a graphical inter-face and content is remotely accessed also through HTTPcommunication. Visual descriptors and similarity measuresimplemented in this work are mostly de ned in the MPEG-7standard, while textual metadata is coded according to theDublin Core speci cations.