How do you have it disabled for the original size image? It's usually the same javascript added to the theme files. However, do note that there are ways around this (i.e. turning of javascript in FF or just doing a "Save As" of the whole page).
I placed this javascript inside the head of my index.php in order to prevent right click. This will disable any image on your page from being right clicked. Hope it helps.
<script type="text/javascript"> <!-- var closeWin="0"; // Do you want to close window (1 for yes 0 for no) var alertVis="1"; // Do you want to alert the visitor (1 for yes 0 for no) var message="Sorry, you can NOT right click here."; // Your no right click message here // Published at: scripts.tropicalpcsolutions.com function detail(){ if(alertVis == "1") alert(message); if(closeWin == "1") self.close(); return false; } function IE() { if (event.button == "2" || event.button == "3"){detail();} } function NS(e) { if (document.layers || (document.getElementById && !document.all)){ if (e.which == "2" || e.which == "3"){ detail();} } } document.onmousedown=IE;document.onmouseup=NS;document.oncontextmenu=new Function("return false");