Image styles are a great tool for working with images in your content, and it comes as part of Drupal 7 core. Drupal generates images for you using pre-built image styles as well as ones that you create. This means that you don't necessarily have to precisely resize an image before uploading it to your Drupal site. This series starts out with the basics of what image styles are and how you can use them, moves on to show some cool tricks, and then finishes up by showing how you preserve all that hard work and re-use it elsewhere.
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This collection covers a topic in Drupal 7 which may or may not be the version you're using. We're keeping this tutorial online as a courtesy to users of Drupal 7, but we consider it archived.