Every WordPress user faces the problem with featured or thumb image when they change the current theme. To adjust with new theme feature sometimes you might need to change the existing featured image size. But if you have already posted thousands of post then it is nearly impossible to change all the existing image size. But good news is in WordPress you can regenerate post thumbnails image by using Regenerate Thumbnails plugin. It allows you to regenerate your post’s thumbnails after changing the thumbnail sizes.
First of all get the plugin from above link and install it. Upon activation you will find the menu under Tools menu as “Regen. Thumbnails”
You can signally regenerate thumbnails from media library too:
You can resize specific multiples images using the checkboxes and the “Bulk Actions” dropdown: