While editing a post or page that has an image in it, click on the image and a toolbox will pop up over it. Click on the pencil icon.
This will bring up an image editing box. Select a new size from the “Size” dropdown box, or create your own custom size by selecting “Custom Size.” In this example we are selecting the “Thumbnail” size, which will reduce our image to 150 x 150 pixels. Once you are finished click the “Update” button in the lower right corner.
The image size has been reduced (in this case to thumbnail size).
