Установите прозрачный фон imageview на Android

There is already a transparent built into Android: R.color.transparent. http://developer.android.com/reference/android/R.color.html#transparent

But I think you may want to make the background of the image that you are placing into the WebView transparent, for example, with a transparent PNG, rather than the ImageView background. If the actual image is not at all see-through then the ImageView background can't be seen through it.