I have a Drupal site that has around 24 image styles defined. The problem I'm having is the files directory seems to be building up a huge amount of files.
If I upload an image, does Drupal automatically created a version for each of the 24 styles, or should it only create the relevant size if it's specifically requested within the code?
Derivative images are created on-the-fly, when they're first requested via HTTP, or when image_style_create_derivative() is invoked manually for the relevant image/style name.
They're not generated en-masse when a file is first uploaded, unless you have a contrib/custom module installed which provides that functionality.
Drupal only creates the file when it's needed.
The way this works is a url is generated, eg. /sites/default/files/styles/public/somestyle/beach.jpg. The first time this url is called, the file doesn't exist.
Due to Drupal's url handeling, Drupal will chop of parts at the end until it has a hit, in this case /sites/default/files/styles will be caught by the hook_menu of the image module. It will get public, somstyle and beach.jpg as parameters.
Based on these parameters, the image module creates the image.
