You can change the image used for it using the following style declaration.
<style name="MyCustomTheme" parent="style/Theme.Holo">
<item name="android:actionOverflowButtonStyle">@style/MyCustomTheme.OverFlow</item>
</style>
<style name="MyCustomTheme.OverFlow">
<item name="android:src">@drawable/my_overflow_image</item>
</style>