In the version that you are using there is a problem related to the RadCaptcha control - the "Refresh New Image" button causes validation to be performed (its CausesValidation property is not set to false). This problem is fixed and you should download the latest official version of the RadControls for ASP.NET AJAX.
If the new DLL does not fix the issue, can you please send us a fully running project that demonstrates the problem? Once we have a closer look, we can provide an adequate solution.
the Telerik team