My featured images don't fit and display properly.

Old Images and Site Performance

If you’re switching your web site with our themes, 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 wordpress repository.

Navigate to Plugins Add New , search the plugin, install and activate it.
After the successfull install you can regenarate 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.