Help understanding how to modify lay out of pages within drupal
I assume you are embedding the images in a block which you then show on all your pages.    

If you reference a background-image from your style sheet as in url(../images/hello.jpg) all works fine because the style sheet, no matter what page you are looking at, references images from the style sheet directory as a base . If you embed an image tag on a page your img url will not be correct for pages of different depths as the page's url changes, and it is the page url that is the base. You have two quick solutions:  


Pathalogic module - it is a text input filter that will rewrite
your embedded anchor and image paths automatically
PHP Filter - It is also a text input filter. Rewrite the path using the l() function, or concatenate it onto $base_url in a PHP snippet. This method is not preferred and support for the PHP Filter has been dropped in Drupal 8.  


And as to the css and theming problems:

Marinelli is using a 960 grid system. There are lots of information on the web available on that and I suggest you read up on that. 

In short, you must add appropriate classes to your images which will then take care of the css on your behalf. To float 3 images horizontally you need to add the following classes to your images.  

image 1. class="alpha grid_4"
image 2. class="grid_4"
image 3. class="grid_4 omega"  

And depending on your use case you might need to do a clearfix on the parent container.