You can do basic resizing of images in WordPress. You can only reduce the dimwnsions of an image – it is not possible to increase the dimensions.
This process only resizes the dimensions (widthxheight in pixels) and is not an image optimizer. While some file size reduction may occur this is not the main function of the WP image resizer. To get optimum file size it is best to reduce the image fiel size outside of WP using a tool like Photoshop.
Locate image to be resized
Go to the Media library and click on the image to resize.
Click on edit image in the Attachment Details window
Change image size in the Attachment Details window
Here we are reducing the size from 1600×1200 to 800×600. Note the resize maintains the aspect ration so your image won’t be distorted. After clicking on scale there may be not noticeable change in the image size but it has been resized. To finish click on the X in the top right of the window.
Changing the size of an image already in a post or page
Simply changing the dimensions of an image will not automatically update the image used ina page or post. WordPress creates multiple sizes of an image when an image is uploaded e.g. Full size (original) Large, medium and Thumbnail. The sizes created depend on the theme settings. The image version that is resized is the ‘Full Size’ version.
So if the image was inserted on a page or post is ‘Large’ then this image is not changed after resizing. You will need to edit the image embedded in the post or page and change it to Full Size – which is now the smaller resized version.