Since raster attachment are being treated as elements in MicroStation V8, it's easier to accidentally delete the attachment.  .

Here are a few options to eliminate this behaviour:

1)  Lock the image.  To do that, select it and then press CTRL+L or select Edit  > Lock.  Note, when the image is locked you'll also be prevented from manipulating or editing it.  To release the lock, ensure the image is selected and press CTRL+M or choose Edit > Unlock.

2)  Lock the level.  Since raster attachments are put on a level, you can also lock the level via the Level Manager. 

3)  Set a preference.  Go to Workspace > Preferences > Raster Manager  and toggle Disable Delete Element Tool on Selected Rasters.  This preference was added in MicroStation V8i.

4)  Key in RASTER DISABLEDELSELECTION SETON. Once this key in is executed you'll need to use the detach tool from Raster Manager to get rid of the selected image.   To revert back to the previous behaviour, key in RASTER DISABLEDELSELECTION SETOFF.   Note, this key in is This key in is available in release 08.09.03.48 and later.