How renderable array for an image with style should look like?
There's nothing specific for images, but there is for any theme function. Your old code using theme() would be equivalent to:

$build = array(
  '#theme' =&gt; 'image_style',
  '#path' =&gt; $image['uri'],
  '#style_name' =&gt; 'product_page_thumbnail',
  '#alt' =&gt; $image['ImageAlt'],
  '#title' =&gt; $image['ImageTitle'],
);


Basically you pass the named variables for the theme function as #-prefixed properties, and the render system will do the rest.