In the current version of RadControls it is not possible to add directly images to the RadHtmlChart as well as to the Axes. If we receive other similar feature requests, we might consider this idea for the future.
You can, however, manually create image for the labels and align it over the RadHtmlChart. There is a good example in KendoUI's Bubble chart demo
that demonstrates how to create a custom legend in a styled Html element and align it over the chart, so that you can use similar approach.
the Telerik team
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