First off, it's not a tag, it's an attribute. Second, while the video @dcristo pointed at does show how to use the idiotic bloated BS known as turdpress to do it, please do us all a favor and ignore at the end of it where dipshit puts a title identical to the alt text on the image. Images should not have titles, that's ALT's job. if you're going to have a title that would likely go on an ANCHOR around just an image -- and if it's going to be the same text, JUST DON'T DO IT!!! Another of those bits of idiotic "HTML, what's that" BS that turdpress dupes people into doing.
Good point. It was probably introduced by SEO spammers, as it was another opportunity to stuff more keywords on the page.
You mean like the opengraph asshattery where they have a "description" and "title" meta separate from the existing HTML ones?