Changing Image Sizes
The Settings Media Screen controls the various settings related to images and other media that are used in writing posts and pages.
You will need php-gd installed on your server to use this feature.
The sizes listed below determine the maximum dimensions in pixels to use when inserting an image into the body of a post.
- Thumbnail size – Enter the Width and Height
- Crop thumbnail to exact dimensions (normally thumbnails are proportional) – check box
- Medium size – Enter the Max Width and Max Height
- Large size – Enter the Max Width and Max Height
Default settings used for during the media upload process when writing a post.
- Organize my uploads into month- and year-based folders – Check this box if you wish to organize your uploads into folders based on the year and the month of the upload. As of 3.5, wp-content/uploads is the default path setting for the uploads folder. Assuming this is your path setting, if you check this box, a file uploaded in June of 2010 would be placed into the wp-content/uploads/2010/06 folder. Note: At the time of the actual upload, WordPress will automatically create the folders, such as wp-content/uploads/2010 and wp-content/uploads/2010/06, as long as your wp-content folder is writeable.
- Click the Save Changes button to ensure any changes you have made to your Settings are saved to your database. Once you click the button, a confirmation text box will appear at the top of the page telling you your settings have been saved.
Article from WordPress.org