Richard,
Did my tutorials not help?
I have so many, surely one would be able to guide you through this task.
To add an image, you first need to find the correct template. Use Webmaster mode to figure out which template you need. I think the footer is customer/bottom.tpl or maybe customer/main/bottom.
Then you need to upload your image somewhere. I recommend skin/ideal_responsive/images if that is your template. Once the image is in that directory, you can reference it in the template like this:
<img src="{$AltImagesDir}/myImage.jpg" />
But if you want it to be responsive, it is best to also add a class( class="myImageClass"). Then you can add some CSS in your skin/ideal_responsive/css/altskin.css:
.myImageClass {
height: auto;
width: 100%;
max-width: 500px;
}
You can use the max-width to scale your image properly for the different screen sizes by including different values in each media query.
Of course, this is the tip of the iceberg. In order to accomplish this thing properly, you need to have a thorough understanding of the web in general, understand file paths, file types, image compression, encoding, permissions, how to unlock X-Cart (if you plan to use the built in editor), FTP or Cpanel, HTML, CSS and the list goes on. That is why I recommend watching tutorials and studying, or just hiring a professional.