The standard way the radio buttons field is rendered on a node add/edit form is that there is a wrapper div with classes:
form-item form-type-radio form-item-mymodule-imglayout-und
and inside there are input and label elements. Where to hook to change this? I would like the wrapper div to have classes:
form-item form-type-radio form-item-mymodule-imglayout-und mycustomclass
And the custom class should be dependent on the option the button represents, or just be sequential: first, second, third, ...etc
My point is to be able to add a distinct big background image to each radio button, depending on the option it represents. Like below - note that each radio button has an associated image, representing its option visually.