Eu criei um Custom ProgressDialog da seguinte forma: Primeiro eu tenho uma lista de animação com 9 Imges seqüenciais

<?xml version="1.0" encoding="utf-8"?>

<item android:drawable="@drawable/icon_progress_dialog_drawable_1" android:duration="150" />
<item android:drawable="@drawable/icon_progress_dialog_drawable_2" android:duration="150" />
<item android:drawable="@drawable/icon_progress_dialog_drawable_3" android:duration="150" />
<item android:drawable="@drawable/icon_progress_dialog_drawable_4" android:duration="150" />
<item android:drawable="@drawable/icon_progress_dialog_drawable_5" android:duration="150" />
<item android:drawable="@drawable/icon_progress_dialog_drawable_6" android:duration="150" />
<item android:drawable="@drawable/icon_progress_dialog_drawable_7" android:duration="150" />
<item android:drawable="@drawable/icon_progress_dialog_drawable_8" android:duration="150" />
<item android:drawable="@drawable/icon_progress_dialog_drawable_9" android:duration="150" />

Então eu tenho um estilo personalizado

 <style name="CustomDialog" parent="@android:style/Theme.Dialog" >
    <item name="android:windowFrame">@null</item>
    <item name="android:windowBackground">@null</item>
    <item name="android:windowIsFloating">true</item>
    <item name="android:windowContentOverlay">@null</item>
    <item name="android:windowTitleStyle">@null</item>      
    <item name="android:windowSoftInputMode">stateUnspecified|adjustPan</item>
    <item name="android:backgroundDimEnabled">false</item>
    <item name="android:gravity">center</item>
    <item name="android:minHeight">70dip</item>
    <item name="android:maxHeight">80dip</item>       

Agora no código eu chamo da seguinte forma

 dialog = new ProgressDialog(KaguaHome.this); 
        dialog.setProgressStyle(R.style.CustomDialog);
        dialog.setIndeterminate(true);
        dialog.setIndeterminateDrawable(getResources().getDrawable(R.drawable.progress_dialog_icon_drawable_animation));
        dialog.show();

O resultado é

No entanto, quero conseguir um progresso Dialog com apenas a imagem (sem fundo branco) e deve ser centralizado, o que devo modificar?