how to strip string from url using rewrite rule

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http://serverfault.com – sometimes my drupal site add extra string to image url which causes the image to be broken. the url is http://mysite.com/sites/default/files/imagecache/list_image_page/%252Fsi... what is the needed rewrite rule to strip the bolded (%252F) part in the above link ie. to be: http://mysite.com/sites/default/files/imagecache/list_image_page/sites/d... I have tried this, but didn't work RewriteCond %{QUERY_STRING} ^(.*)\%252Fsites(.*)$ RewriteRule %{REQUEST_URI} %1sites%2 (HowTos)