Raster Manager is the name of the principal module in MicroStation that is responsible for opening and visualizing raster imagery in MicroStation.
Using Raster Manager you can open and display images of various formats, from monochrome to full color. Single images, or groups of images, can be set to display in one or more DGN file views. MicroStation's drawing and annotation tools still can be used in views that display raster images.
You can modify the location, display order, and various other settings of previously attached raster image files. Changes can be made via the Element Information dialog , the Modify tool, or the attachment information section of the Raster Manager dialog. Additionally, the Warp tool lets you fit (warp) a raster image to a particular shape.
When you display a raster image file in a DGN file, it is attached in a Raster Reference Attachment. When a raster attachment is modified with Raster Manager, no changes are made to the original file, just to its attachment information in the DGN file. To make permanent changes to the image, please refer to Bentley Descartes: http://www.bentley.com/en-US/Products/Bentley+Descartes/