You cannot mix expression language inside scriptlet. You should use scriptlet only or expression language only.
To fix your current error, retrieve the attribute from request (or wherever it is stored):
<% String i = ((CForm)request.getAttribute("cForm")).getId(); %>
To really solve your problem: stop using scriplets at all, keep it all in expression language:
<img src="/supportcenter/cServlet?sampleImg=${cForm.cId}" width="300px" />