Not sure what you mean addons to put above actionbars. Where above action bars?
For pictures, there are requirements regarding size and formatting and so on. One of the guides linked describes how to make photos into usable textures in wow using photoshop (or in your case, Gimp, which is the same thing for what you need here).
The bar thing is called a tooltip and you need an addon to move it or change it. Here are some tooltip addons you can choose from: