I had some what the same problem, only I had a black background. Ok, at the moment I don't have fcp in front of me, so I'm only giving you some vague descriptions. If you right click on the clip in your timeline there should be a menu (there for example the option speed), there should be something here under speed called: composite mode. Now I don't exactly know which one it was but try some options here till you get the right one. If knothing works try a black background and try it again.
I even once gave a picture a bright green background and keyed it out (that's an option).
GOod luck.