There isn't a way to disable the captcha validation, because it is an important security feature that prevents spam.
Regarding the exact problem - is only the image gray in your case, or the entire captcha does not render? I am asking this because in a webfarm/garden scenario the captcha often renders a gray image because the in process session state is not shared among all instances. More information about this issue is available here: http://www.telerik.com/help/aspnet-ajax/captcha-webfarm-and-webgarden.html
If this does not solve your situation could you provide more details on the problem, for example a simple page that shows a captcha that does not render so we can investigate this case?
the Telerik team
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