It could be that it's not a bug in image.module itself, but there is an easy fix.

On line 635 the var $label should be just text, eg 'thumbnail'. But in some situations, it's an array Array( [0] => 'thumbnail' ).

By putting this on line 635 the issue can be solved: if(is_array($label)) $label = $label[0];

-----

Some background info:
The page is a view using image_gallery module. The field is 'Image gallery: Latest image'.