You need your app to be registered as an app that knows how to handle images, you can do that using the intent-filters build argument.
Something like this should do the trick:
android.xintent_filter=<intent-filter>
<action android:name="android.intent.action.SEND" />
<category android:name="android.intent.category.DEFAULT" />
<data android:mimeType="image/*" />
</intent-filter>