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According to theme_image_style page you tried wrong parameters,

as an example, your code should be something like the following snippet

$variables = array(
        'style_name' => 'thumbnail',
        'path' => $image_fields[LANGUAGE_NONE][0]['uri'],
        'width' => $image_fields[LANGUAGE_NONE][0]['width'],
        'height' => $image_fields[LANGUAGE_NONE][0]['height']
);
print theme_image_style($variables);

and in short

print theme('image_style', array(
        'style_name' => 'thumbnail',
        'path' => $image_fields[LANGUAGE_NONE][0]['uri'],
));