Does Google use the image alt tag the same as anchor text for external back links or does it only use the alt tag for internal optimization?
The alt tag can be used as anchor text for both internal and external use. However ideally when using the alt tag the alt text should be used to describe the image, not the external link.
Below is two examples
<a href="http://www.example.com"><img src="cows.jpg" alt="cows in field"></a>
<a href="http://www.example.com"><img src="cows.jpg" alt="example"></a>
While 1 or 2 can be used technically the correct one is number one because an alt tag is originally designed for accessibility such as blind people, impaired vision and so on. Matt Cutts from Google has said in several videos that its good practice to use alt tag on external or internal links rather than using the
indent:9999em method using CSS, so assuming you just care about Google and not about accessiblity then you should proceed to use anchor text in the alt tag rather than describing the image.
The purpose of alt attributes is to provide a description of the contents of an image file. One of the most cited uses of alt attributes is to provide text for visitors who can’t see images in their browsers. If a viewer can see your images, alt attributes will also show when the user hovers over your image.
In addition to ensuring the content in your image is communicated whether the image shows or not, effectively using alt text can also be a great way to improve your site’s placement in search engine result pages because it allows the images on your pages to rank in image search results.