Starting today you can draw UML diagrams in Astah (Community) and display the diagrams in Confluence. This way you have a good alternative for the built-in Gliffy editor that uses Flash and works in your browser. Create your UML diagram in Astah, and attach your Astah diagram to the page:
Then modify the page in Confluence and start editing the page. Using the Macro Browser (Insert->Other Macro's, or start typing a { ) you can insert your diagram and your diagram is on the wiki.
Using the macro you don't need to export the diagram to PNG/JPEG!