var yourImage = new Image(),
yourImage.src ='http://your.site.com/your_image.png';
data = {
labels: ['one', 'two', 'three'],
datasets: [{
label: 'Label1',
pointRadius: [0,0,0,0,20],
pointHoverRadius: [0,0,0,0,20],
pointHitRadius: [0,0,0,0,20],
pointStyle: ['', '', '', '', yourImage],
data: [1,2,3]
}, {
label: 'label2',
pointRadius: [0,0,0,0,20],
pointHoverRadius: [0,0,0,0,20],
pointHitRadius: [0,0,0,0,20],
pointStyle: ['', '', '', '', yourImage],
data: [1,2,3]
}]