As its name indicates, this is the first image found on your page.
You would use it generally within an iterator.
For the purpose of the demonstration, we have included this image to act as our first image
With the page image tag, you can then show it with the following syntax
<page-image first width="400" linking="default"/>
How to see the value of the first image?
In the image tab of the metadata manager, you can also see the value of the first image.