Preload image 

Method 1
Basic concept. Load image when page is loaded

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Method 2
Improve method 1. Create function

function preload(images) {
    if (document.images) {
        var i = 0;
        var imageArray = new Array();
        imageArray = images.split(',');
        var imageObj = new Image();
        for(i=0; i<=imageArray.length-1; i++) {
            //document.write('');// Write to page (uncomment to check images)
            imageObj.src=images[i];
        }
    }
}

usage

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Method 3
Create 1 pixel image and hiding it somewhere in your page

Method 4
similiar like method 3 but hide it using css

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