2. Free Online Picture Dictionaries

There are a lot of more or less challenging Free Online Picture Dictionaries, partly interactive and with audio explanations too.

Scientific: + Audio
http://visual.merriam-webster.com/
Very interesting and detailed:
http://www.infovisual.info/
With audio:
http://www.eflnet.com/vocab/index.php
Innovative:
http://www.washjeff.edu/CAPL/Good explanations:
http://www.pidic.com/

Kids/Beginners:
http://www.enchantedlearning.com/Dictionary.html
http://giraffian.com/pictionary/
http://iteslj.org/v/ei/
Interactive:
http://www.parapal-online.co.uk/picture_dict/index.html