How to change the image style for image preview in node-add/edit form?
If you are using the Media module, I couldn't find a way to change it so it doesn't use the "Media thumbnail" image style. But what you could do is edit the "Media thumbnail" image style by going to /admin/config/media/image-styles/edit/media_thumbnail and clicking on the override button which will let you edit it to your liking. (Tested on Drupal 7)



If you're not using the Media module, you can by going to your content type's Manage Fields and clicking on edit for your image field. Scroll down a bit until you see Preview image style