Always be careful with taking the user outside the application. Users get distracted and don't come back.
Let's assume it's an app that puts a photo for each month. The user will find natural that if the photos are not in the phone library, the user needs to go and find ones either online or on their computer and pass them to the phone.
Depending on how the embedded browser is implemented and how much hassle it is, the likelihood of the user to come back to the app changes. Unless it's beautifully integrated, I would suggest expecting the user to have all the images in the library. The user will be prepared and it will be faster to create the calendar.
Think about how would the app look with facebook, google images, etc embedded. If the solution doesn't look good or reduces the screen size considerable, I would suggest not taking this approach.