[html] HTML img tag: title attribute vs. alt attribute?

That's because they serve different purposes and they both should be used not just one over the other.

The "alt" is for what you guys already said, so you can see what's the image it's all about if the image can't be displayed (for whatever reason), it also allows visually impaired people to understand what's the image about.

The "title" attribute is the correct one to show the tooltip with a title for the image.