Try to mention your style like this then this will shown as a dialog and transparent..
<style name="DialogTheme" parent="android:Theme.Dialog">
<!-- No backgrounds, titles or window float -->
<item name="android:windowBackground">@null</item>
<item name="android:windowIsTranslucent">true</item>
</style>
In your activity apply theme like this..
<activity
android:name="com.afbb.smartlock.activitys.PreviewImagesActivity"
android:theme="@style/DialogTheme" >
</activity>