I just created a gallery and for some reason, I’m unable to edit/remove the title that shows up on hover. I’m able to do it individually for pictures I insert into individual posts or pages, but for the ones in the gallery.
That isn’t something that can be removed by CSS. It is the “title” set in the image code.
Go to media > library, edit the image and remove the “title” and then save changes. Do that for each image. WordPress software, when uploading images, will put the base file name as the title automatically.
The gallery is a different animal. I had not tried to remove the title on gallery images, but I aways include the title and alt attributes on image for accessibility and SEO reasons. Not to mention it prevents XHTML validation errors since the alt and title tags are required for validation.