There isn't any
alt text associated with an image. That's not part of its metadata. The image metadata includes the file name, and, optionally, things like shooting information (exposure, aperture), and user-entered things like a title, a comment, some keywords, etc. The
alt text is simply a requirement of web page layout - an image always has to include some text that the browser can use if the image can't be loaded. This text isn't part of the image - it's just what text the web page creator
chooses to attach to the tag for an image. For example:
<img src="mydog.jpg" alt="a picture of a dog" title="My Dog">
This will show
mydog.jpg on the page. When the user hovers on the image, the popup will show
My Dog. If the image can't be loaded for some reason, the space where the image should be will show the words,
a picture of a dog, in that space. Screen readers for the blind will also "speak" the
alt text, since the blind person can't see the picture. The
alt text can be anything the web page writer chooses. It's not an attribute of the
mydog.jpg file itself.