Unfortunately there is no way to fulfil your requirements using css alone as far as I know for all browsers. It is not recommended for the title/alt tags to be left empty due to the seo loss from not making you website friendly enough for the visually impaired.
So your alternative is to either embrace the tooltips and allow your seo to give a poignant company speech addressing the plight of the visually challenged; another option would be to completely stop using the 'title' element of images and instead use 'alt' (as apple.com and google.com have done), or to use some kind of a jquery. The downside of following in the footsteps of google and apple is that google doesn't follow the seo rules it creates, and apple refuses to acknowledge that there is more than 2 browsers.
Here is one solution using jquery ( a small piece ) which hides the tooltips. But you will have to adapt it to your own needs. Here is a working example with the css, jquery, html ... http://jsbin.com/agucu/3/edit
If you look around you will see how many websites have the same functionality that you just never noticed before (yahoo menus, parts of ebay, a good number of logos as well of top tier web sites.