Hi,
There is not a separate property in page settings for a tooltip (aka title) and if we used link text for the title it would be bad for accessibility because screen readers would read both the link text and the title and this double reading is annoying for users of screen readers. Even if the title is a little different from the link text its still pretty annnoying on a large menu to have it read both so we don't use it by default on the main menu.
We actually use the title property (since it is not used for title on menu items) to store a custom css class so that customer per item css classes on menu items are supported this way.
However you can change it to not use the title property for custom css class by setting
UseMenuTooltipForCustomCss="false" on the mojoMenu or mojoTreeView in theme.skin
Then "I think" it would use whatever you put for the custom menu css class in page settings as title (or alt if using images) but if it is blank it will not render a title.
I generally don't recommend using images in menu items for large menus unless you are doing it by css with css sprites and using the images as background so that only one image is needed for it. When you use spearate images with <img elements it makes the page have lots of extra http requests to load all those images which is bad for performance. Its ok maybe for small sites with only a few pages in the menu.
Hope that helps,
Joe