Re: what insert a image or where modify tiki-index.php?
It depends on where you want the image to go or how you want to modify the page. Basically, the layout is determined by the theme's tiki.tpl file. Some themes use the default templates/tiki.tpl file and some have their own, like templates/styles/(stylename)/tiki.tpl.
You can add an image tag to this file directly, but tiki.tpl already presents a "full" page, that is, with tiki-top_bar.tpl at the top, the three columns across the middle, and tiki-bot_bar.tpl at the bottom, so any image added to tiki.tpl is going to affect how those parts are displayed.
You can also use the theme's CSS to change div sizes and positions and give them new background images, etc. Check out themes.tikiwiki.org for more information, or post again here about what you want to do specifically.
-- Gary - themes.tw.o