Plupload widget type not appearing
You need a combination of modules:


https://www.drupal.org/project/filefield_sources_plupload
https://www.drupal.org/project/filefield_sources
https://www.drupal.org/project/plupload


I have intentionally left these as URLs as these module are often confused between each other.

In the content type which you wish to use the plupload widget: manage fields > images > edit.
Scroll down until you see "File Sources", expand and select "Advanced Upload Widget (Plupload).

These basic steps should give you the option to use plupload when uploading images. You can further change the setting to allow a specific number of images etc.