var cCurPrefix;
var aImageIds = ['img1','img2'];
function ChgImgSizes() {
     var cNewPrefix = document.body.clientWidth > 1200 ? 'large_$1' : 'small_$1';
     var i, o, c;
     if (cCurPrefix != cNewPrefix) {
          cCurPrefix = cNewPrefix;
          for (i in aImageIds) {
               o = document.getElementById(aImageIds[i]);
               o.src = o.src.replace(/[^\/]*_([^_]*)$/, cNewPrefix);
          }
     }