You can change the default image size of your image uploads! You are not limited to the default sizes set by Wordpress for the thumbnail, medium, or large images.
Go to the Admin panel and choose Settings — Media. In Image Sizes, you will see Thumbnail size, Medium size, Large size. In the Width or Maximum Width section, type in the size that you want. I normally leave Height or Maximum Height to 0. Then click Save Settings.
Open the page or post where you’d like to upload the image. Put your cursor at the spot. Click the Upload/Insert icon. You get the Add Media box. Choose the location of the image, then scroll all the way down to the Size section. Choose the size that you want, and you will see that it populates with the new size setting. Click Insert into Post.
I want my blog post images to fill the entire width of the content area, so I set Medium to 600px. Here is a screenshot of what it looks like.
If you had already uploaded an image into the page or post, you will need to erase the image and follow the above steps. The image won’t change its size automatically when you change the media settings.