How can I resize a picture I've already posted? I can click and drag the corner of the picture and it resizes the picture, but only horizontally, so the picture looks really squashed. I can also go into the advanced settings and manually change the height and width in pixels but the same still happens. What am I doing wrong?
You need to change the size in settings to the new one. Then you have to regenerate the images to that new size because when you change the size within WordPress they are still saved as the previous size. For that you can use a plugin like : Regenerate Thumbnails
After you have run the generate you can simply delete the plugin after.
Description of the plugin: