The primary function of easy-thumbnails is to dynamically create thumbnails
based on a source image.  So whenever a thumbnail does not exist or if the
source was modified more recently than the existing thumbnail, a new thumbnail
is generated (and saved).  Thumbnail aliases can be defined, providing
predefined thumbnail options.  This also allows for generation of thumbnails
when the source image is uploaded.

See also: https://github.com/SmileyChris/easy-thumbnails