Call a DLL from another atl DLL

Hi all, i am new to ATL and COM programming but I have to develop an
ATL DLL which called internally another COM DLL in order to do some
stuff (this DLL object is aimed to create an animated gif )...
What I have done is put an import directive in my cpp file as this :

#import "ImgMake.dll" no_namespace

and I implement a method in my COM DLL which create the ImgMake object
and add multiple frame image to it with a for loop as this :

        CoInitialize(NULL);
        IImgMakePtr im ;

    for (item = m_itemList.begin(); item != m_itemList.end(); item++)
    {
        //... m_img_fileName is a BSTR member of an item
        IBitmapPtr b ;
                b.CreateFromFile(m_img_fileName);
                im->Add(b);
    }
        im->Save(); // --> Where I have the error
       CoUninitialize();

If I have only one item in my object list evry thing going fine, else
I have an error as I try to call the save method. (I have tryed also
to assign to b a NULL value before using it)
Can you help me with this?