It can occur that changing the theme of your WordPress blog can cause old uploaded images to glitch and display improperly. Using the Regenerate Thumbnails plugin can resolve this problem.
Install the Regenerate Thumbnails Plugin
Configuring The Regenerate Thumbnails Plugin
- Log in to the WordPress Dashboard.
- Click "Tools" and then "Regen. Thumbnails".
- Click "Regenerate All Thumbnails".
The new image sizes will be generated in the sizes defined by the Settings>Media page.