Actually any changes ``default_options`` keys, like this one might be quite disastrous.

Because thumbnail hash is computed based on source, geometry_string and options (see ``_get_thumbnail_filename``),
changing any of those will force a new thumbnail to be created when get_thumbnail is called. 
This is what we usually want, when we change source and dimensions. 
But changing options, e.g. THUMBNAIL_QUALITY, or, adding ``alternative_resolutions``, like here,
will make ALL exisitng thumbnails obsolete.

This is typical thundering herd problem. If you have, say, thousands of large images and some traffic on your site,
simultaneous thumbnail generation might effectively put the whole site down. This what might happen after changing
backend defaults.

We need to tackle this problem before merging this into master.