Default html appear when creating new Post or Page? Please help!

In the past I have created splash pages for clients by creating a fairly blank custom .php template (with some CSS in it) and then created a home page with some html tables to grid out a picture, button links, logos, and text.

I’m now working on a multi-user site for a client who wants users to be able to create posts that are more image heavy. Think magazine layouts with a complex table grid like a quilt.

I want to implement the same function if I can, creating a custom .php template so a user could choose from a few templates in the right sidebar. My personal issue is I don’t know how to make that default html table grid come up when the template is selected.

Does anyone know if it’s possible to have anything appear by default when you add new posts or pages? What part of the code should I poke around?

Any help would be appreciated!