What UIImage should I return from UIActivity activityImage?

I created a subclass of UIActivity to represent a custom action in an Activity Activity in my application. However, I can’t get him to show my icon.

I tried several different images with typical icon sizes, for example:

- (UIImage *)activityImage { 
    return [UIImage imageNamed:@"my_app_icon.png"]; 
}

but I just get a gray square and my icon is not showing at all.

EDIT: The real problem was that my Xcode documents were out of date, so I had no information that the maximum image size is very limited.