You need to indicate that your app can open image files. This is done in the Info.plist:
<key>CFBundleDocumentTypes</key>
<array>
<dict>
<key>CFBundleTypeName</key>
<string>Images</string>
<key>LSHandlerRank</key>
<string>Alternate</string>
<key>LSItemContentTypes</key>
<array>
<string>public.image</string>
</array>
</dict>
</array>
The above will allow your app to appear in the "open in" menu for images. If your app is selected, your app will be launched or brought to the foreground and the application:openURL:sourceApplication:annotation:
delegate method will be called. The URL will contain a reference to the image file.