Change record status: 
Introduced in branch: 

This commit Renamed the Image system's 'path' attribute to 'uri', to maintain consistency with this similar change introduced within the File API.

This change is reflected in image objects. Contrib modules which referenced the 'path' key of an image object in D7 need to reference the corresponding 'uri' key in D8.

You know you're hitting this if calls to theme_image() return a broken link pointing to the site's root URL.

Drupal 7

return theme('image', array('path' => 'misc/arrow-asc.png', 'width' => 13, 'height' => 13, 'alt' => t('sort ascending'), 'title' => t('sort ascending')));

Drupal 8

return theme('image', array('uri' => 'core/misc/arrow-asc.png', 'width' => 13, 'height' => 13, 'alt' => t('sort ascending'), 'title' => t('sort ascending')));
Module developers
Updates Done (doc team, etc.)
Online documentation: 
Not done
Theming guide: 
Not done
Module developer documentation: 
Not done
Examples project: 
Not done
Coder Review: 
Not done
Coder Upgrade: 
Not done
Other updates done

There was a suggestion in the original issue that perhaps the key should be 'src' instead. A new issue should be opened to discuss this proposal.