I would like to insert an image before credit cart payment, now there is only on the paypal payment (i insert an image to clear the question)
Every time you want to customize Magento and you find an example from the default codebase, you found half of the solution.
Since you have seen that the Paypal payment method includes an image, have you reviewed how that image has been implemented?
You can do that by inspecting the image in the browser, and then looking for the image filename in the codebase.
Have you reviewed that implementation?