Why do Android Buttons render differently than ImageButtons?

Question

Why does my Android layout work when I use Buttons with a background, but break when I use ImageButtons? I have read through the documentation and cannot figure out why they render differently. I see that one is a TextView and that one is an ImageView but I assume a dp is a dp regardless of the context. Between the two layouts the only thing that I have changed is "ImageButton" to "Button" and "src" to "background".

Working layout:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent" 
    android:orientation="vertical"
    android:background="@drawable/background">

. . .

<Button 
    android:contentDescription="@string/login_button_facebook"
    android:layout_width="200dp"
    android:layout_height="45dp"
    android:background="@drawable/first_thought_menu_2_facebook"
    android:onClick="loginWithFacebook"        
    />

<Button 
    android:contentDescription="@string/login_button_email"
    android:layout_width="200dp"
    android:layout_height="45dp"
    android:background="@drawable/first_thought_menu_3_email"
    android:onClick="loginWithEmail"        
    />

<Button 
    android:contentDescription="@string/login_button_anonymous"
    android:layout_width="200dp"
    android:layout_height="45dp"
    android:background="@drawable/first_thought_menu_4_anonymous"
    android:onClick="loginWithAnonymous"        
    />            

. . .   

</LinearLayout>

Picture of working layout:

Broken layout:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent" 
    android:orientation="vertical"
    android:background="@drawable/background">

. . .

<ImageButton 
    android:contentDescription="@string/login_button_facebook"
    android:layout_width="200dp"
    android:layout_height="45dp"
    android:src="@drawable/first_thought_menu_2_facebook"
    android:onClick="loginWithFacebook"        
    />

<ImageButton 
    android:contentDescription="@string/login_button_email"
    android:layout_width="200dp"
    android:layout_height="45dp"
    android:src="@drawable/first_thought_menu_3_email"
    android:onClick="loginWithEmail"        
    />

<ImageButton 
    android:contentDescription="@string/login_button_anonymous"
    android:layout_width="200dp"
    android:layout_height="45dp"
    android:src="@drawable/first_thought_menu_4_anonymous"
    android:onClick="loginWithAnonymous"        
    />          

. . .   

</LinearLayout> 

Picture of broken layout:

Answer 1

Use android:background instead of src.