It seems like your labels are shifting left and right when the image is added. This could be coming from the IntelligentLabelsEnabled property being set to true. You may be able to solve your problem by setting this property to false.
You may also want to try binding your image to the second series on the chart. You would then see the images line up at the far end of the RadChart as opposed to seeing the labels at random points in the chart.
If this is not what you need, you may want to consider using a RadHTMLChart. This is a newer Chart available for Telerik's RadControls and it has much more flexibility when manipulating the labels of your Chart. You may want to give this demo
a look to see if a RadHTMLChart may better suit your needs.