There's an entire practice that says it's a bad idea to have inline functions/styles.
Taking into account you already have an ID for your button, consider
JS
var myvar=15;
function init(){
document.getElementById('EditBanner').onclick=function(){EditBanner(myvar);};
}
window.onload=init;
HTML
<input id="EditBanner" type="button" value="Edit Image" />
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