Overlapping Views in Android

Android handles transparency across views and drawables (including PNG images) natively, so the scenario you describe (a partially transparent ImageView in front of a Gallery) is certainly possible.

If you're having problems it may be related to either the layout or your image. I've replicated the layout you describe and successfully achieved the effect you're after. Here's the exact layout I used.

<RelativeLayout 
  xmlns:android="http://schemas.android.com/apk/res/android"
  android:id="@+id/gallerylayout"
  android:layout_width="fill_parent"
  android:layout_height="fill_parent">
  <Gallery
    android:id="@+id/overview"
    android:layout_width="fill_parent" 
    android:layout_height="wrap_content"
  />
  <ImageView
    android:id="@+id/navigmaske"
    android:background="#0000"      
    android:src="@drawable/navigmask"
    android:scaleType="fitXY"
    android:layout_alignTop="@id/overview"
    android:layout_alignBottom="@id/overview"
    android:layout_width="fill_parent" 
    android:layout_height="wrap_content"
  />
</RelativeLayout>

Note that I've changed the parent RelativeLayout to a height and width of fill_parent as is generally what you want for a main Activity. Then I've aligned the top and bottom of the ImageView to the top and bottom of the Gallery to ensure it's centered in front of it.

I've also explicitly set the background of the ImageView to be transparent.

As for the image drawable itself, if you put the PNG file somewhere for me to look at I can use it in my project and see if it's responsible.

Also, take a look at FrameLayout, that's how the Camera's Gallery application implements the Zoom buttons overlay.

If you want to add you custom Overlay screen on Layout, you can create a Custom Linear Layout and get control of drawing and key events. You can my tutorial- Overlay on Android Layout- http://prasanta-paul.blogspot.com/2010/08/overlay-on-android-layout.html

Visible gallery changes visibility which is how you get the gallery over other view overlap. the Home sample app has some good examples of this technique.

The simples way arround is to put -40dp margin at the buttom of the top imageview