Im not sure which edit button your are referring to, but i don't think it is the right one.
when you select the form tag in design view, the property inspector will show 3 buttons:
to change the look of the captcha element, click the Edit Design Button (or click Edit Form then Customize CSS)
in the Edit Design window, change the initial select list to Form Elements, then in the secondary select list choose Captcha. This window will allow you to edit the look of the captcha image.