Plug in one
Name: Regenerate Thumbnails
This plugin goes back and makes the thumbnails for all of your image uploads, individual image uploads, or specific multiple image uploads. This will be a great plug in because with the amount of images our websites will have this does a lot of work for us. When I was making a website with my company we had this exact problem, the thumbnails were not the size we wanted them to be, we wanted to see the whole image in the thumbnail. This plug-in really helped with that.
It has 4.7 stars out of 5. Some say it has not worked for them but most users say it is great and even will improve the speed of your site. It is a free plug in and there are forums on wordpress.org that you can look through if you need support. It is compatible with up to wordpress 4.6.1/ It was last updated 3 months ago and has over 1 million active installs.