I want to swap background and foreground of my raster image.

If your TIF image background is black and the foreground is white and you need to reverse the foreground and background.  If you have Descartes highlight the raster image line in Raster Manager and to the right of the invert icon along the bottom of raster manager you'll see the boxes that have the colors of the foreground and background.  There is a very small icon that's an arrow to the top right of these 2 boxes.  If you hover over it you should get "swap colors".  Hit this and it'll swap the background and foreground colors.  Then do a File > Save As set file type to CIT and create a new CIT image.  This option is no in MicroStation.

 

In Workspace > Preference > Raster Manager > Default Attributes > Set Default Attributes > Color change Invert to Invert Display.  This will invert your raster images when they are attached automatically.   The problem is that all of your raster images will be inverted when you attach them.  You might consider creating a New Userspace that you can use instead of Untitled when you go into MicroStation and you know that you are going to be working with raster images that need to be attached inverted. 

 

Another option would be when you attach your raster image in the Raster Attachment Options box in the Color section change Invert to be Invert Display.  Your raster image would then be inverted in your DGN file.

 

If you want your file to be printed like it shows on the screen try doing Print > Settings > Raster Options and mark Print monochrome raster as-is.