Corners Banner
The module is absolutely simple and immediate to configure, once installed you can select a folder of your choice to pick images, and assign them to module instances.
You can choose which corner of your site the image will display, with additional info like image title and link for users click.
The module have advanced functionality to detect your image dimensions, so that you will have to do nothing to center images to fit exactly to the corners with no bad margins.
Simply install module with your Joomla! installer and once published and chosen the image of your interest it will appear exactly at the corner of your page.
Moreover using Joomla! modules you can insert more than one corners banner in the same page with different position, or choose to show different banner in different pages.