This paper addresses the general problem of how building a distributed image search&retrieval system that copes with metadata heterogeneity. Firstly, We analyze the usage of metadata in current image repositories, the different metadata schemas for image description and their semantic relationships. After, we provide a general classification for the different approaches which provide a unified interface to search images hosted in different systems without degrading query expressiveness. This paper analyzes how these approaches can be implemented on top of the latest standards in the area, ISO/IEC 15938-12 (MPEG Query Format) for the query interface interoperability and ISO/IEC 24800 (JPSearch) for the definition and management of translations between metadata schemas. Finally, we provide insights into an example of a real distributed image search&retrieval system which provides real time access to Flickr, Picassa and Panoramio.