Is it possible to add a bitmap to a Workbook in Discoverer Desktop and then let this bitmap visible on Discoverer Plus ?

Currently this feature is available in Desktop only, and not implemented yet in PLUS version So
when saving a bitmap in Desktop it will not appear in PLUS.
There is an Enhancement Request 3996567 for this issue. (BITMAP IMAGES LOGO IN PLUS/VIEWER)
This feature is approved for future release by discoverer product management but at the time of writing this document there is no a fix date .

However, a logo could be added in PLUS to the applet window for all users regardless of the
workbooks, this could be done through the Enterprise manager
Application Control :
Middle tier >> Discoverer >> Discoverer Plus >> Logo >> File then browse
to the desired logo file

for Discoverer viewer, you can also add a logo to the main browser window for all users and
regardless of any workbook:
this could be done through the Enterprise manager Application Control :
Middle tier >> Discoverer >> Discoverer Viewer >> customization >> select
Layout then press GO >> Logo >> File then browse to the desired logo file

for more information, please review :
Oracle® Business Intelligence Discoverer Configuration Guide
10g Release 2 (10.1.2.1) for Microsoft Windows and Solaris Operating System (SPARC)
ch: 9 Customizing OracleBI Discoverer
9.2.3 How to change the default Discoverer Viewer layout
http://download-uk.oracle.com/docs/html/B13918_03/customize.htm#BABFHDJI