In Drupal 7 there is a theme('image_style') which is similar to theme('imagecache')
$hero_image = array(
'style_name' => 'case_study_teaser',
'path' => $image['uri'],
'width' => '',
'height' => '',
'alt' => $image['alt'],
'title' => $image['title'],
There is also a very handy little function in Drupal 7.
$img_url = 'public://myimagefolder/imgfilename.jpg'; // the orig image uri
$style = 'thumbnail'; // or any other custom image style you've created via /admin/config/media/image-styles
<img src="<?php print image_style_url($style, $img_url) ?>>
The image_style_url() function takes the URI and converts into the relevant URL for the image style provided. Note, you need to use public:// rather than the base url of your site for this function to work.
The great thing is that if the image hasn't yet been created then Drupal does it automatically!