Остановилась на этом варианте:
std::auto_ptrTMemoryStream> pms(new TMemoryStream);
if(DBImage1->Picture->Bitmap != NULL)
{
DBImage1->Picture->Bitmap->SaveToStream(pms.get());
FDQuery1->Params->ParamByName("foto")->LoadFromStream(pms.get(), ftBlob);
}
else
{
FDQuery1->Params->ParamByName("foto")->AsBlob = NULL;
}