I don't think it is so violent that it should be rejected. But since it has been rejected, do you need to have that comic bundled with the app?

It looks like you are marketing your app as a comic reader, not a service that provides comics, so I'd think that even giving out a free comic would not be integral to your product.

So I'm sure this must be frustrating, but I'd just take it out, or just replace it with something undeniably tame.