Adaptive Image is a simple module you can use to make your images responsive in Drupal 7 by using Image Styles and applying a simple effect.
Say you are working on your responsive theme in Drupal 7 but your images formatted with the core image styling do not resize with the browser. You may want to make a slideshow or put them in a View someway else.
Well, there’s the Adaptive Image module at your disposal for Drupal 7. Quite simple to use too.
Install and enable the module. Then when you add the image style you use for your image - after the effects you would normally add - also add the newly installed Adaptive Effect. Set the resolution breakpoints separated by a comma and Save. You can then test it out and see! Jut resize your browser.
Hope this helps.