use background
attribute for your ImageView
Since the background needs to be circular use an shape xml
<?xml version="1.0" encoding="utf-8"?>
<shape
xmlns:android="http://schemas.android.com/apk/res/android"
android:shape="oval">
<solid
android:color="#A9A9A9"/> <!-- you can set the color you want here -->
<!-- you can also define size if required-->
</shape>
then use it as a background
android:background="@drawable/your_file_id"
This way when the image is showed it'll hide the background color.. if you don't set an image the background will be shown
Update
In order to avoid problems with partially transparent images and ratio remomve background when setting image resource