The word 'toad' has been used to mean anyone hateful since the 1500s. (The earliest recorded use of the word in that way was in reference to the French!) Lying, of course, is generally considered wrong, so calling someone 'a lying toad' is just a means of letting them know you know they're not telling the truth and that you find that thoroughly unpleasant.
"....American FReepers flood the British media with messages of support for Prime Minister Blair. This will probably have the effect of convincing my bloody-minded compatriots to kick out that lieing toad after eight long years of plaguing us".
"....he is a "LIEING COMMIE TOAD" and the toad wouldn't know the truth if it hit him in the face. He will say what ever it takes to get him in the Pres. office. He will cheat, lie and steal or what ever to get his way..."