As you are returning the bitmap to be used by the caller - then it is the callers responsibility to dispose the object when they're finished with it.
By using the 'using' statement you've already done all you can to ensure that the graphics object is properly disposed. In theory you should be fine - I wouldn;t worry about that unless it becomes a specific problem - then come back and ask how to solve it.
With regard to your edit - unless 'writerOut.WriteVideoFrame' disposes of the bimtap when its finished with it then this will cause a problem.
you could either do :
var bm = Merge( ... );
writerOut.WriteVideoFrame(bm);
bm.Dispose();
or
using(var bm=merge(...))
writerOut.WriteVideoFrame(bm)