what file do i look at

where do i fine thi file index.php?main page=index&cPath=page=4a
thanks for any help

There is no single file that holds that page - the page's HTML output is built on the fly by a combination of many different PHP files working together.

If you rephrase your question to what you want to do with the page, perhaps we can suggest an appropriate file to look at.

If you are looking to make changes to your main page of the cart you can do so here:

Admin>Tools>Define Pages Editor>Select_Page>Define_Main_Page

That will affect the welcome/introductory home page center content. It will not affect any category or product listing content, which is what the referenced address points to. We just need more information to give concrete advice.

guess what i'd like to do is to put a back ground pic on
some pages
like some thing for fire ?

Then you will generally be wanting to modify the stylesheet. There are hundreds of threads of people asking about this in one form or another, with various answers and information given.