Child themes inherit the functionality of another theme, so not so much overwriting it as to enhancing and adding to what is already existent in the parent theme. You can read more about this in the
WordPress codex. If you're creating your own child theme, we have more documentation relating to this
here so you have an idea of what is involved, but basically if you are looking to make any code changes, that is, changes that are outside of what you can configure within the admin dashboard, then we recommend that you do create a child theme. This will make things a lot easier when you're updating the theme itself as you won't wipe out all of your modifications. Our themes are not encrypted, so when it comes to "can I make XYZ changes?" yes you technically can, if you have the coding knowledge to do so. All of the files are included when you purchase the theme so you can access sizes etc then. The
Vantage customer showcase can give you some good examples of customizations that other customers have made with the default theme. The theme is not designed to use a standard page as the homepage, so doing so may require customization to get all the normal homepage links/features working correctly. Yes you could print an image used in a listing.
Vantage is designed to allow you to charge users to place listings, but the blog function itself is not set up for that as it's generally accessed by the admin only. You also cannot charge users to register by default. Thanks.