Disposing of Bitmaps Properly

I've been reading many blog posts/forums on the issue of Android bitmaps. In my app, I load a moderate amount of Bitmaps which are placed in ImageView's. I am using the methods described in the documentation about "Loading large bitmaps efficiently". When I load each of my bitmaps I can see the native heap increase in size by < 1MB. Not a huge problem until I have to load quite a few.

1. How do you know if I have a memory leak? I can see the heap size increasing while loading the bitmaps and even after I dispose of all of the resources (ie. bitmap, imageview etc) in the OnDestroy() and Finish() the activity, the heap size never goes down. The problem is i'm not sure when the GC is supposed to reclaim the memory? Is it only when the OS needs more resources? Or does it happen periodically? If so, how often? Even after I finish the activity and wait awhile, I keep checking the heap size and it never decreases.

2. How do you properly dispose of a bitmap? Is simply wrapping the bitmap in a using statement like I have below good enough? Or do I have to look out for some lingering reference to a Context or something?

Basically, I have a custom view (derives from LinearLayout) which in turns has an ImageView. The user can take a photo or load it from the gallery and when finished, the imageview is populated with the image they took/selected. I have pasted some code below. Does anyone have any ideas where/if there is a problem?

//DecodeImageFromFile uses the InSampleSize to reduce the size
using(Bitmap bitmap = DecodeImageFromFile(ImageFilePath, _Global.DeviceWidth, _Global.DeviceHeight))
{
     if (bitmap != null)
    {

     _ImageSelected = new ImageView(Context) {LayoutParameters = Utils.GeneratetLinearLayoutParams(5,5,10,10)};
     _ImageSelected.SetImageBitmap(bitmap);
     _ImageSelected.SetAdjustViewBounds(true);
         Context.RunOnUiThread(() => _Layout.AddView(_ImageSelected));

     ImageAdded = true;
     _Global.AppLayer.LogMemory();

    }
}

and In the OnDestroy() of the activity, I go through each custom view I created and call the method on the custom view that does this

if(_ImageSelected != null)
{
      _Layout.RemoveView(_ImageSelected);
  if (_ImageSelected.Drawable != null) {
    _ImageSelected.Drawable.Dispose ();
  }
  _ImageSelected.SetImageBitmap(null);
  _ImageSelected.Dispose();
  _ImageSelected = null;
}