Prompt a message when image size is too small

gistfile1.txt

var d = new Date();
var mth = d.getMonth() + 1;
var str1;
            
if (mth == 11) {
    str1 = '//localhost/wordpress/wp-content/uploads/2015/11/';
}
else if (mth == 12) {
    str1 = '//localhost/wordpress/wp-content/uploads/2015/12/';
}
                       
var str2 = str1.concat(file.name);

var img = new Image();
img.onload = function () {
    if (this.width < 700) {
        alert('The size of the photo is too small.');
    }
}
img.src = str2;