A group of research projects based at HP-Labs Bristol, the University of Bristol (England) and ARKive (a new large multimedia database project focused on the worlds biodiversity based in the United Kingdom) are working to develop a flexible model for the indexing of multimedia collections that allows users to annotate content utilizing extensible controlled vocabularies. As part of the educationally focused ARKive-ERA project, a series of models for user annotation have been developed. One example is that of university lecturers and researchers studying a particular type of animal behavior. They may wish to identify all relevant images or video of that particular behavior and annotate them as good illustrations of aspects of that behavior. However, significant issues arise over, for example, the validation of information, access control and the use of such annotations by the resource discovery tools. The paper explores these and other issues and problems involved, and explains how the various models can help provide solutions to key problems and thus meet the needs of a diverse range of communities of interest, thereby adding significant value to online multimedia collections. (Contains 22 references.) (Author/MES)