Hello @Martin Thompson 

Hope you are doing well, 

In website builder can we remove a selection of these images, 

-->first to click on edit option on particular image and after that you can see all images are shown in blocks. 

-->Once, you hover any image you can see the top of the right side of image delete option and you can delete it. 

here you can see image: 

-->you want to delete bulk images then you can find backend side attachment.
-->inside attachment on search area select group type Mime Type and filter type Stored.
-->you can see images/jpeg and image/png and then you can delete it.


here you can see image:

I hope this will help you.

Thanks & Regards,
Email: odoo@aktivsoftware.com
Skype: kalpeshmaheshwari

Took me a while but I found it. Go to Settings and scroll to the bottom. Active the developper mode. Then you will be able to find attachments in the settings and can delete them from there :)