The image parameter handler will be automatically invoked when that template is displayed.

This basically grabs the request parameter with the image name, looks it up in the database, and returns all the information about that image, separated by pipes (| s), just like the spec said.

For example, if you only have 60 frames in one of your source files, and you specify 75 as the maximum image frames parameter, your final rendered result will have blank entries in part of the image.