open your bitmap editing tool bar, use the magic wand, select the "white" and hit delete.
This is about the only thing you can do in flexi related to bitmap editing (except maybe cropping) so, I'd suggest photoshop instead. If you do select & delete a dirty white background, remember if you have text on that background, and the middles of the letters have that same off-white... then you have to select each middle too... and often in Flexi, you can't even see the problem until you print the file... so again, photoshop is the place to fix this sort of thing if you have it.