If you are looking for images of animals or plants, it is often quite sufficient to use Google,
http://www.google.co.uk/ and click on Images - this link will take you there.
Alternatively, try one of the many specialist sites. This list has been provided by Harry Green, but let me know if have more to add:
http://www.coleopterist.org.uk/ Web site of The Coleopterist. Lots of beetle pictures
http://home.hccnet.nl/mp.van.veen/therevidae/therevidae.html#Pandivirilia Therevid key - link is to the Pandivirilia
http://www.microcosmos.nl/beet1gal.htm Gallery of water beetle pictures
http://www.zin.ru/ANIMALIA/COLEOPTERA/eng/all_fams.htm Illustrated index to families of beetles
http://www.biol.uni.wroc.pl/cassidae/European%20Chrysomelidae/index.htm Chrysomelid beetles
http://www.britishbugs.org.uk/index.html British plant bugs
http://www.ukwildlife.bravehost.com/article/shieldbugs/shieldbugs.html British shield bugs
http://www.freewebs.com/worldofsnails/britishsnails.htm British snails
http://www.cerambycoidea.com/titles/alexander2002.pdf Keith Alexander EN Research booklet on species using decaying wood.
http://www.fugleognatur.dk/wildaboutdenmark/ Lots of pictures of insects etc