see if this makes sense...
take your (placed high res link) image file, draw a gradient fill box (default settings ok for now), select both objects, go to transparency palate and choose make opacity mask. you should see the gradient fill becomes a transparency faded mask. you may have to tweak the gradient fill to get the desired results. another tip: in the transparency palate tab there is the selection tool for the object and the mask, be aware of which object layer is active and what object is being adjusted. after you play with a bit you'll understand. then click back to base object before moving on to edit any other objects (if transparency mask layer is active, no other objects can be edited).