Hi,
Before designing the form you just select an image field and insert into your page. Then add an image which will be your background image.
Now, go to the object pallete,select Field Tab and go to the Presence section and select the presence as hidden. This means initially there will not be any images whenever user will open the form.
Now, in the click event of radio button just write the below code.
For Yes,
ImageField.presence = visible;
For No,
ImageField.presence = hidden;
Regards,
Manjeet