Auto select Radio group from function

Hi
Use a selection of buttons to autofil a form with different values depending on the button pressed.
Works no problem with text and checkbox fields but can't get it to work on a radio group.

Here's one of the functions which works fine apart from the MenuImage field which is a radio group.

function CheckDefault()
{
form1.MenuSize_Width.value = 241;      //Text Field
form1.MenuColor_Normal.value = '#efefef'     //Text Field
form1.MenuImage.checked = 'CMSMenu/images/img1.gif'       // Radio Group. This is the problem one!
form1.ShowAll.checked = true     //Checkbox
}

Any ideas?