It is possible for you to create your own skin folder and override templates.
If you just want to change images, you can just copy the original file to your skin and add some simple code, wrapped in an "if" so that this only happen on grouped product pages
in a rough sketch, something like
<?php if ($_product->isGrouped()): ?><?php endif; ?>
will only execute whats in between for grouped products, leaving other types unchanged.
If your changes are more extensive, it will be more manageabl to define in the layout.xml file a section for the grouped product type that makes the site use different templates or layout logic. That way you can keep the originals clean and it is also better for performance.
To give an example on my site I want to loop over the simple products and show attributes that belong to them, on the grouped product details page - I am changing two templates but also using the "translate" option to override the title
<block type="catalog/product_view_type_grouped" name="product.info.availability.snippet" as="product_type_availability_snippet" template="catalog/product/view/type/availability/snippet/grouped.phtml" />
<block type="catalog/product_view_description" name="product.description" as="description" template="catalog/product/view/type/groupeddescription.phtml">
<action method="setTitle" translate="value"><value>Description</value></action>
This requires you to figure out reference names of the different areas of the pages etc. which will take longer, so if you only want to add an image banner you are probably OK just adding an "if"
I hope this helps start you thinking