How to call preprocess functions from my custom module
Looking at template_preprocess_image_formatter, I would try the following code below. 

Replace mymodule with the machine name of the module. This assume foo is the machine name of your image formatter.  

mymodule_preprocess_image(&amp;$variables) {
  /* processing logic */
}

mymodule_preprocess_image_formatter_foo(&amp;$variables) {
  /* processing logic */
}


I also recommend reviewing Drupal 8's Core Image module as it has some of the files that call the hooks you need. 


image.field.inc contains template_preprocess_image_formatter
template_preprocess_image