Many themes take advantage of Wordpress built-in media handler. Some even want you to configure the sizes to exact specifications. Doing so isn’t difficult – here’s where you find the setting in two short steps:
1. Go to the administration area (yoursiteaddress.com/wp-admin)
2. Open the Settings tab and click on Media
Set your sizes and click on save. You’re done!
Feel free to experiment with the different sizes to see what works for you. The different image sizes are generated when you upload an image, so if you want to generate already uploaded images in the new sizes you have to upload them again.
