  The profusion of online digital images presents new challenges for image
indexing. Images have always been problematic to describe and catalogue due to
lack of inherent textual data and ambiguity of meaning. An alternative to
time-consuming professionally-applied metadata has been sought in the form of
tags, simple keywords that form a flat structure known as distributed
classification, or more popularly as a folksonomy.
  This research aims to increase understanding of why people tag and how
effective they find it for searching, using as the focus. Open-ended
questionnaires were sent out to members of the photo-sharing website Flickr,
with the opportunity to post comments to an online discussion space. There is
also a systematic comparison between a tag-based system and a more traditional
controlled vocabulary, to test out the claims made regarding the suitability of
tagging for searching and browsing. For this purpose Flickr has been compared
with Getty Images using a series of test themes.
  The small number of people who replied to the questionnaire gave detailed
answers that confirmed several assertions made about tags: they are accepted
despite their flaws (sloppiness and potential for inaccuracy) because they
serve their purpose to a satisfactory level. Some answers challenged the
assumption that tagging is only done for personal benefit. The search
comparison found that while Getty allows highly specific queries and logical
semantic links, Flickr is more flexible and better placed to deal with subtle
concepts. The overall conclusion is that tags achieve most when used in
conjunction with groupings of people with a shared interest.
