Instead of using alpha(since it fades everything inside the parent) you can use background colour with transparency.
For 0.7 alpha, which means 70% white you can use (B3 is 70% value):
android:background="#B3FFFFFF"
So you will have a 30% transparent background and your ImageView will be visible as normal.
If you insist doing it by alpha, you must exlude your ImageView
from the View
which you set alpha value.