Custom checkbox image android

Question

Create a drawable checkbox selector:

<?xml version="1.0" encoding="utf-8"?>
<selector xmlns:android="http://schemas.android.com/apk/res/android" >
    <item android:drawable="@drawable/checkbox" 
          android:state_checked="false"/>
    <item android:drawable="@drawable/checkboxselected" 
          android:state_checked="true"/>
    <item android:drawable="@drawable/checkbox"/>    
</selector>

Make sure your checkbox is like this android:button="@drawable/checkbox_selector"

<CheckBox
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:button="@drawable/checkbox_selector"
    android:text="CheckBox"
    android:textAppearance="?android:attr/textAppearanceLarge"
    android:textColor="@color/Black" />

Checkboxes being children of Button you can just give your checkbox a background image with several states as described here, under "Button style":

...and exemplified here:

Copy the btn_check.xml from android-sdk/platforms/android-#/data/res/drawable to your project's drawable folder and change the 'on' and 'off' image states to your custom images.

Then your xml will just need android:button="@drawable/btn_check"

<CheckBox
    android:button="@drawable/btn_check"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:checked="true" />

If you want to use different default android icons, you can use android:button="@android:drawable/..."

res/drawable/day_selector.xml

    <?xml version="1.0" encoding="utf-8"?>
    <selector xmlns:android="http://schemas.android.com/apk/res/android" >
        <item android:drawable="@drawable/dayselectionunselected"
              android:state_checked="false"/>
        <item android:drawable="@drawable/daysselectionselected"
              android:state_checked="true"/>
        <item android:drawable="@drawable/dayselectionunselected"/>
    </selector>

res/layout/my_layout.xml

<CheckBox
    android:id="@+id/check"
    android:layout_width="39dp"
    android:layout_height="39dp"
    android:background="@drawable/day_selector"
    android:button="@null"
    android:gravity="center"
    android:text="S"
    android:textColor="@color/black"
    android:textSize="12sp" />

If you have Android open source code, you can find the styles definition under:
src/frameworks/base/core/res/res/values

<style name="Widget.CompoundButton.CheckBox">
    <item name="android:background">
        @android:drawable/btn_check_label_background
    </item>
    <item name="android:button">
        ?android:attr/listChoiceIndicatorMultiple
    </item>
</style>

Based on the Enselic and Rahul answers.

It works for me (before and after API 21):

<CheckBox
    android:id="@+id/checkbox"
    android:layout_width="40dp"
    android:layout_height="40dp"
    android:text=""
    android:gravity="center"

    android:background="@drawable/checkbox_selector"
    android:button="@null"
    app:buttonCompat="@null" />