In one of my recent project I had a task to create a content type with a select list field of all image styles. But how can we get all image styles in Drupal? In Drupal there is an option to create different image styles using a module 'image'. Image module allows users to upload images into Drupal with proper permissions along with this its thumbnails and additional sizes are created automatically. Image module also gives an image_styles() function that can be used to get all image styles and their settings.
In Drupal 7 Image module offers an image field which makes available an image widget for file fields that can be added to content types. This will allow for upload of images to the image field which can then later be processed by the image module and be presented using the different image styles as configured. This means that image module should have the ability to process files uploaded using this field.