the easiest method is to put your thumbnails in an unprotected directory, then put the full size images in a new directory under the thumbs, call it like images, then put a htaccess file in the images dir, use the referring url method and you can then specify which urls you want to be able to access the images in the htaccess.
this is how I have one of mine
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RewriteEngine On
RewriteCond %{HTTP_REFERER} !^
http://([a-z0-9-]+\.)*landofvenus.com/ [NC]
RewriteCond %{HTTP_REFERER} !^
http://([a-z0-9-]+\.)*venuscam.com/ [NC]
RewriteCond %{HTTP_REFERER} !^
http://([a-z0-9-]+\.)*amateurmuscle.com/ [NC]
RewriteCond %{HTTP_REFERER} !^
http://([a-z0-9-]+\.)*girlshouse.com/ [NC]
RewriteCond %{HTTP_REFERER} !^
http://([a-z0-9-]+\.)*amateurbarbara.com/ [NC]
RewriteCond %{HTTP_REFERER} !^
http://([a-z0-9-]+\.)*young-amateur-pussy.com/ [NC]
RewriteCond %{HTTP_REFERER} !^
http://([a-z0-9-]+\.)*misschianti.com/ [NC]
RewriteCond %{HTTP_REFERER} !^
http://([a-z0-9-]+\.)*aaronsbabes.com/ [NC]
RewriteCond %{HTTP_REFERER} !^
http://([a-z0-9-]+\.)*amy2please.com/ [NC]
RewriteCond %{HTTP_REFERER} !^
http://([a-z0-9-]+\.)*ninasboobs.com/ [NC]
RewriteCond %{HTTP_REFERER} !^
http://([a-z0-9-]+\.)*sweetshanice.com/ [NC]
RewriteCond %{HTTP_REFERER} !^
http://([a-z0-9-]+\.)*model-gaby.com/ [NC]
RewriteCond %{HTTP_REFERER} !^
http://([a-z0-9-]+\.)*vcomm.net/ [NC]
RewriteCond %{HTTP_REFERER} !^
http://([a-z0-9-]+\.)*dirtydakota.com/ [NC]
RewriteCond %{HTTP_REFERER} !^
http://([a-z0-9-]+\.)*msmuscle.com/ [NC]
RewriteCond %{HTTP_REFERER} !^
http://([a-z0-9-]+\.)*bicherokee.com/ [NC]
RewriteCond %{HTTP_REFERER} !^
http://([a-z0-9-]+\.)*jadabangcock.com/ [NC]
RewriteCond %{HTTP_REFERER} !^
http://([a-z0-9-]+\.)*hottdancers.com/ [NC]
RewriteCond %{HTTP_REFERER} !^
http://([a-z0-9-]+\.)*yummyangelica/ [NC]
RewriteCond %{HTTP_REFERER} !^
http://([a-z0-9-]+\.)*lacykitty.com/ [NC]
RewriteCond %{HTTP_REFERER} !^
http://([a-z0-9-]+\.)*princesslauraleigh.com/ [NC]
RewriteCond %{HTTP_REFERER} !^
http://([a-z0-9-]+\.)*risquewife.com/ [NC]
RewriteCond %{HTTP_REFERER} !^
http://([a-z0-9-]+\.)*pinupkittens.com/ [NC]
RewriteCond %{HTTP_REFERER} !^
http://([a-z0-9-]+\.)*sexytexasamateurs.com/ [NC]
RewriteCond %{HTTP_REFERER} !^
http://([a-z0-9-]+\.)*hurricanebridget.com/ [NC]
RewriteCond %{HTTP_REFERER} !^
http://([a-z0-9-]+\.)*nude-female-muscle.com/ [NC]
RewriteCond %{HTTP_REFERER} !^
http://([a-z0-9-]+\.)*nude-fitness-model.com/ [NC]
RewriteCond %{HTTP_REFERER} !^
http://([a-z0-9-]+\.)*showgirlsbackstage.com/ [NC]
RewriteCond %{HTTP_REFERER} !^
http://([a-z0-9-]+\.)*daphniesdreamhouse.com/ [NC]
RewriteCond %{HTTP_REFERER} !^
http://([a-z0-9-]+\.)*alldancers.com/ [NC]
RewriteCond %{HTTP_REFERER} !^
http://([a-z0-9-]+\.)*amateurbarbara.com/ [NC]
RewriteRule /*
http://www.landofvenus.com/index.html [L,R]
-------------------