What you likely want is a transparency. However if the banner is just one image file, doing this will grey out the whole banner, and I assume what you want to do is just grey out the fishes, so that they are less obvious.
If you created this banner yourself using the individual fish and loaf images and dragged them in, you could highlight all of these images (by holding down CTRL and clicking them individually) and in the effects pain there is a slider (which will be at 100%) you can change this to any value (say try 25%) which will make all the images slightly 'see-through' which may be the effect you want.
However, if this is just one image then doing this would just make the whole banner half see through. If you have the individual fish/loaf images then you could create it again and do this, if not you could export just the banner as a gif, choosing this blue background as the transparency and export it this way. Then you could bring it back in onto a new blue background and make this transparent.
Sorry to go off on tangents but I am at work just now, with no access to fireworks, but if you do not get anywhere with the above then reply, and I can maybe get a copy of the image off you to get a better idea of what you are working with, and how to get the effect you want.
"The only stupid question is the one that doesn't get asked