You can't edit images using WebEdit on Tableau Server 9.3 and below.
(In Tableau 10 there is the facility to edit dashboards, but I dont think that you will be able to replace images either)
If you need to make changes to the Dashboard you will have to make the changes in Desktop and republish to Server
Thanks for your answer. Do you now if is possible change the image in the path of tomcat or maybe in the path of Tableau Server files?
It is not possible as the images are part of the report itself.
Thanks for your answer. It not possible change images like shapes in Tableau Server with Edit option.