Possibility to add content on panel pages

This is a common question and the best answer I can give is as follows:

1.) Panels does not provide you an easy way to make "global" additions of panel panes to pages. This is to say the case you are describing, where you add an image to the sidebar of all pages, isn't really possible in the Panels world.

2.) Panels, however, does play nice with random theme regions so it would be possible to have a global sidebar implemented with a mini panel and exposed to a sidebar region through the block.module. Check out http://drupal.org/project/panels_mini_ipe if you want to use the IPE with it.

3.) Panels also has a "Reusable Content" idea so you could make a reusable item that you want in your sidebar and you would just need to add it to all the pages.