fixes #230 scope issue in TimberImageHelper

After upgrading Timber to Version 0.18.1 (hosted on WPEngine), ran in to this error when trying to delete photos from the media library (the same issue as in #230):

>PHP Fatal error: Cannot access self:: when no class scope is active in /nas/wp/www/cluster-1088/ups/wp-content/plugins/timber-library/functions/timber-image-helper.php on line 10, referer: http://ups.wpengine.com/wp-admin/upload.php?post_mime_type=image

With the help of @jarednova solved with these changes (it was a scoping issue, hooked actions are not in the class context so 'self' is not available, or at least doesn't refer to TimberImageHelper)