Easily regenerate thumbnails in your WordPress site after changing the thumbnail sizes from Dashboard -> Settings -> Media.
Sometimes when you change the size of thumbnail from Settings -> Media page your old thumbnails takes an odd look. For example: Suppose you changed the thumbnail size from 100px to 250px, then your old thumbnails would look somewhat stretched and that leaves a bad impression on your readers.
Using Regenerate Thumbnails plugin you can generate thumbnails for your posts after changing their dimensions. Install and activate it, then visit your Dashboard -> Tools -> Regenerate Thumbnail for generating new ones. Also if you like to regenerate selected thumbnails the you can do it from Media Library by checking the images and choosing the option Regenerate Thumbnails from Bulk Actions dropdown as shown in the second screenshot.