page of post thumbnails – want hover effect to reveal post title

It’s not alt, it’s title that creates a pop-up on mouse-over.

LOL! Gotta keep your priorities straight after all! Is “your” team winning?

Yep, those are the title=”XXX” attributes assigned to each image. To get rid of them, you’ll have to go through and remove that from each image. I think fancier pop-ups are usually done with Javascript — there are probably plug-ins to do something like that — search the plug-ins page.