Okay there are several ways you can do this:
1. Looking at the code in the file you have posted, you probably will use both. The index.html is where you put the image tag and any class you assign to it. The default.css is where you will put the parameters for the class you assign to the tag.
I would of created an example but you did not include a css file so that we know what we are looking at in the index file you posted. I am assuming it is a two column page?
2. Or simply find the code for the column you want to add your image tag in the index file. Then use the image tag to put simple variables for the image in there. example: Let's say you wanted to put an image under the title Lorem Ipsum. You could do this:
<div class="single_column">
<h3>Lorem Ipsum</h3>
<img scr="mypic.jpg" width="200" border="2" align="center">
<p>Curabitur et velit libero. Duis viverra arcu eget elit venenatis id interdum dui semper. Sed gravida ante sed neque fringilla posuere. Cras consequat urna sed tellus gravida auctor. Vestibulum tortor enim, placerat eu gravida et, semper at nulla. , quis rhoncus odio iaculis et. Etiam eu tellus mauris. Vestibulum tortor enim, placerat eu gravida et, semper at nulla. Morbi fermentum purus lorem, in vestibulum turpis. Suspendisse potenti. Duis nibh enim, ullamcorper id laoreet.</p>
</div>
This should produce an image under "Lorem Ipsum" and before the Paragraph. Just remember that if you did not have the <p> tag before the paragrah, you might want to at least include a <br /> tag to force the text to go below the picture.
This example is assuming that you don't have the image tag defined in your css for positioning and such. If you did this example might not work. It is hard to tell from your example because there is no css file to show us what the settings are in it.
Since you are a newbee - Janys gives good advice to look at the templates that come with the software. It is a great learning tool. Just pick one for fun and pick it apart and see what the code does.
Have fun,
Kim