The fuzzball is too fuzzy to fill all of the small white areas with the gray. And even if that'd work, it still wouldnt solve it if the pictures we are handling are more complex. I just want to completely get rid of the white background so I can make two pictures overlap.
Yes, I tried that too. It did remove one of the white parts but removing all of them manually would be a pain (at least if the fuzzball is larger). But this does solve the problem in cases where there isn't any white area inside of the object, so I have to thank you for the information.