Can I launch the Photos app from the iPhone app? Like launching mail?
[[UIApplication sharedApplication] openURL:url];
The Photos app does not register any URL patterns that other apps can use. In other system applications (for example, Safari) there is a fairly well-established "save image" template, as a result of which the image is placed in the user's camera roll; I would recommend just sticking to the expected behavior and perhaps use an alert to direct the user to see where your application has been exported.
This article looks like it might be useful? Retrieving Images from iPhone