Use Form::setFieldAttribute(). Typically, this is done in the model's getForm() method. However, if it's cosmetic only, it can be done in the view too.
In /administrator/com_image/models/forms/image.xml (outside of a <fieldset> is fine):
Your $rating variable contains the following fields:
content_id, rating_sum, rating_count and lastip
You can see this by examining the database table #__content_rating.
The ratings are not saved into the database table with an user ID so there is no way to achieve what you want, sorry.
When you change it to if ($userIP != ($rating->userId) the result ...
I'm not sure why but your website has some duplicated CSS IDs e.g.:
CSS ID selectors should be unique.
Also, CSS selectors should only have hyphens, underscores, letters and numbers.
You may need to update the CSS IDs in the page builder and make the corresponding changes in the custom ...