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var d = app.activeDocument;
var pgs = d.pages;

for (var i  = 0; i < pgs.length; i++){
        
        var rects = pgs[i].rectangles;
        
        
        for (var j = 0; j < rects.length; j++){
            
            if(rects[j].images.length >0){
                img  = rects[j].images[0];
                    var colVal = img.fillColor.colorValue;
                if(colVal.length <= 3){
                    img.fillColor = d.swatches.item(3);
                    alert("changed rgb to swatch black");
                    }
                
                }
            
            }
    
    
    }