default image block classname

Hello, I am trying to use JS to update a default classname which is added to an Image Block.
When the user adds an new image block in their editor, a default class is added.
Additionally, I need to be able to update this default class, so if a user updates a select drop down in the editor, the default class added to future image blocks is updated.

I have tried using Styles, and registering default styles for image blocks which seems to be on the right track, but I am unable to update the default style for future image blocks with JS.

Is there another way of doing this?