I am linking images to my dashboard using action URL. However the image does not resize automatically in Tableau Server (though it does resize automatically in Tableau Desktop and Tableau Reader).
I thought I was doing something wrong but it seems it is not possible to have this auto-resize feature without some complicated (but clever ) workaround (http://community.tableausoftware.com/thread/135676?q=image%20resize%20web)
So here it is as a Suggestion. Please have Tableau Server behave the same way as Tableau Desktop with the resizing of images inside web page object.