1) Show me you understood the problem. - You are a photographer so the size and the quality of the images are, naturally, very important. That means you had to override wp standard resolution for thumbnails and other image sizes. Now you have an issue with previews in the new resolution.
2) I will check the settings of your "regenerate thumbnails" plugin, I will see if good results can be achieved through that plugin, and if not, I would find an alternative or edit that plugin.
Best regards,
Adam Radivojevic