If you’re switching your web site with Breena, it needs to be prepared new images that fit with theme. Also it will boost your web site performance.
So easy way to do that is using a plugin named as Regenerate Thumbnails. You can install this plugin by using WP repository.
Navigate to Plugins Add New , search the plugin, install and activate it.
After the successful install you can regenerate your image thumbnails. Navigate to Tools › Regen. Thumbnails then click Regerarate All Thumbnails button. It will take time depends on your media assets. So wait till to the process is finished.