Default Image For Custom Field

Hello,

I would like to how do I add some default custom fields for new posts?

For example my template has the option of adding feature and screen images, which you can enter an image url into the custom field “Value”.

So how would I set wordpress to display a default image for both of those fields unless one is entered when submitting a new post?

Also I would like those two custom fields to automatically show when you submit a new post like the custom field “aktt_notify_twitter” as per this screenshot of my wordpress back end , so how would I do that please?

Thanks,
Nathaniel

P.S.
I am using the theme gamezine if that makes a difference.