Graphical mechanisms for spatially organizing personal media data could enable users to fruitfully apply their mental models. This paper introduces Semantic Regions, an innovative way for users to construct mental models by drawing regions on 2D space and specifying the semantics for each region. Then users can apply personal ontologies to personal media data using the fling-and-flock metaphor. This allows photos (or other personal media) to be dragged to the display and automatically grouped according to time, geography, family trees, groups of friends, or other conceptual maps. The prototype implementation for Semantic Regions, MediaFinder, was validated with two usability tests for usage and construction of user-defined mental models. Applications and extensions are discussed.