You may have encountered problems when trying to switch to a new theme where the previous theme displayed thumbnails next to the excerpt of the post. The problem can occur in such a way that the uploaded new images are looking good after activating the theme, but the old thumbnails are ugly and distorted. To solve it, you’ll need to regenerate thumbnails or new image sizes in WordPress.
When you upload an image to WordPress, it saves in the upload folder; this creates different sizes by default. In a previous article, we showed you how to create additional image sizes in WordPress. There are many WordPress themes which are taking advantage of this feature for defining new image sizes. But these new image sizes can be applied only on the uploaded images after activating the theme. To fix this issue you have to generate new sizes for all previously uploaded images, a process referred to as regenerating thumbnails.
To begin, install and activate the Regenerate Thumbnails plugin to generate images with newly defined sizes. Then go to Tools > Regen. Thumbnails.
After clicking the regenerate thumbnail button, the new image sizes will start to generate as it is defined by your theme or in your Settings > Media page. To regenerate image sizes for individual or multiple images, you need to go Media > Library.
Original image size will not be affected after regenerating image sizes. It also does not delete the previously created image sizes; instead, it creates additional copies of images as it is defined. You can delete the leftover sizes at your own leisure, if you like, or save them if you prefer. Either way, you should have access to appropriately sized images at this point.