I do not know any alt tag, but I know about the img alt attribute.
The alt attribute for an img is used when the browser is unable to display your image for any reason. If you use the HTML5 standard then you must provide an alt attribute as it is in the specification.
You should describe your image in the alt attribute. So if you have an image that displays a blue chair in 1905 in Munich, then you can write "A picture of a blue chair in Munich, in 1905" as the alt attribute so that Google will know what the image is about and can rank it accordingly.