before you setup mapviewer themes for display, please make sure you have a valid georaster:
1. you don't need to create the georaster dml triggers any more if you are using 11gR1 or later. The triggers are created and maintained automatically when you create a georaster table.
2. for the geometry metadata, the SDO_LB and SDO_UB values you entered could be too small. In general, you simply input any much larger boundary to cover all (old or upcoming) geometries completely in that table.
3. run sdo_geor.validateGeoRaster to see if you georaster image is valid.
4. run the GeoRasterViewer or MapBuilder (no need of a defined theme) to directly display the image to make sure the image looks ok.
Then you can try to define the themes and use MapViewer for web apps.
For additional information while previewing in MapBuilder, set the log level to finest (Tools/Preferences).
If you still are experiencing problems, post the log information and also your theme XML definition.