Hello,
you can't (and never should) change Alfresco source files via an extension module build by the SDK. The Alfresco classes are bundled in their own JAR and can't be overriden via an AMP addon (with some trickery, you can include classes that take precedence over the Alfresco classes but this wouldn't be a stable solution).
Your project builds the class just fine and it is most likely deployed in the final WAR, but it is simply not used as the original class is there and has precedence.
Also, it is highly discouraged to still be customizing the Explorer UI. That client is deprecated in 4.2 and has been removed with 5.0, so you are investing effort into something that either locks you to an old / obsolete version of Alfresco or is a work for the trash bin.
In order to customize the PageTag class you would have to rebuild the alfresco-web-client JAR from source (including your changes). This is not something the SDK helps you with as this is not related to any standard development use cases. The difficulty in customizing the Explorer UI with regards to something as simple as an image is also one of the reasons why it has been deprecated and the newer Share client had been introduced before removal of Explorer.
Regards
Axel