The default browser, email program and music player is stored in a GConf key (you can adjust these via the default applications dialog), but there is no such option for an image viewer. Therefore, the dash behaves the following way: It checks for Shotwell, gThumb, F-Spot and EOG and chooses the first available option. If none of these are installed, the icon is not displayed. So you shouldn't have been able to start Picasa from the dash home screen in the first places, because Unity as (atm) no way of knowing whether it is installed or not. Clicking the image viewer icon on the dash home screen should have opened Shotwell (or another of the mentioned alternatives) both before and after you uninstalled Picasa.