Previous Theme added thumbnail class to every image

I’ve been using ‘the Bootstrap’ theme for a couple of years and it has added a “thumbnail” class to every image.

I would like to start using a new theme now and want to remove the class from all the retrospective images. Is there a way to do this please?

(without manually editing the HTML on every image/page:-)

So an example of what the theme has automatically added is:

<a class="thumbnail" href="https://swindon-bonsai.co.uk/2016/02/29/swindon-winter-image-show-2016/img_3138-4/" rel="attachment wp-att-23337"><img class="alignnone wp-image-23337 size-large" src="https://swindon-bonsai.co.uk/wp-content/uploads/2016/02/IMG_3138-1024x683.jpg" width="770" height="514" /></a>