set default pref for image blocking.

bug 279782, r=dwitte, sr=darin

modules/libpref/src/init/all.js

@@ -613,7 +613,7 @@ pref("network.automatic-ntlm-auth.trusted-uris", "");
 // post to the server specified by this pref.
 pref("network.hosts.nntp_server",           "news.mozilla.org");
 
-pref("network.image.imageBehavior",         0); // 0-Accept, 1-dontAcceptForeign, 2-dontUse
+pref("permissions.default.image",           1); // 1-Accept, 2-Deny, 3-dontAcceptForeign
 pref("network.image.warnAboutImages",       false);
 pref("network.proxy.autoconfig_url",        "");
 pref("network.proxy.type",                  0);