When is image “description” ever used?

When you upload an image you have the option of adding a title and a description. The title you used is added to the title tag when you post an image (as an aside, in the “Insert/edit image” popup it’s then called “Image description”).

I’d like to know if and when the image description you add when you upload the image is used?

I’d also rather use my image description in the title tag when I post images, so I’ve started leaving the description box blank and simply adding a descriptive title.