Is there a tools attribute for ImageView content in Android?

It's just straightforward like for the TextView. Just use tools:src to set the image.

<ImageView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    tools:src="@drawable/placeholder" />

Use this for random real sample images:

<ImageView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:scaleType="centerCrop"
    tools:src="@tools:sample/backgrounds/scenic" />