This paper presents graphical tools that facilitate manual building of formal semantic annotations for digital objects. These tools are intended to be integrated in digital object (DO) management systems requiring semantic metadata (e.g. precise indexation, comment, categorization, certification). We define a multidimensional graphical model of semantic annotations that specifically allows contextualization of annotations, and we propose a methodology for building such annotations. The graph-based formalism used (derived from Conceptual Graphs), provides graphical representations that users can easily understand, and are furthermore logically founded. A graphical generic API implementing elements of the SG family has been developed. CoGUI consists of three specialized tools: a tool for defining a CG ontology, another for creating annotations based on a CG ontology, and finally a tool that uses the CoGITaNT platform, for querying an annotated DO base.