How to Control Media on Your Blog with WordPress
Under the Settings menu in WordPress, you will find the Media link; click the Media link and the Media Settings page opens. This allows you to control the media for your WordPress blog.
On the Media Settings page, you can configure the options for how your image files (graphics and photos) are resized for use in your blog.
The first set of options on the Media Settings page deals with images. WordPress automatically resizes your images for you in three different sizes. The dimensions are referenced in pixels by width and then height. (For example: 150 x 150 means 150 pixels in width by 150 pixels in height.)
Thumbnail Size: The default is 150 x 150; enter the width and height of your choice. Select the Crop Thumbnail to Exact Dimensions check box to resize the thumbnail exactly to the width and height you specified. Deselect this check box and WordPress resizes the image proportionally.
Medium Size: The default is 300 x 300; enter the width and height numbers of your choice.
Large Size: The default is 1024 x 1024; enter the width and height numbers of your choice.
Finally, the last set of options on the Media Settings page is the Uploading Files section. By default, the Organize my Uploads into Month- and Year-Based Folders check box is selected and WordPress organizes your uploaded files in folders by month and by year. Deselect this check box if you do not want WordPress to organize your files by month and year.
Be sure to click the Save Changes button to save your configurations!
Be sure to click the Save Changes button after you set all your options on the Privacy Settings page to make the changes take effect.