The ‘?w=640’ on the end of the image file is not a bad thing, it just tells the browser what size to display.
If you want to modify the size of the image try these methods:
(1) The simplest method is to edit the image on the page. When you’re in the Visual Editor of the page, click on your image. An editor bar should come up over the image. Click the pen icon and select the size you want displayed.
Or, you can
(2) Go to Appearance -> Customize and open the page with the issue.
Open up the tab at the bottom called ‘Additional CSS’.
After selecting the page from your Menu, ‘right click’ on the image, ‘Inspect’ the image and create a custom style for that image. Put the new style you created into the ‘Additional CSS’ and save.
Be careful because if you modify ‘img’ it will modify all images for your site. It is best to see what or contains the ‘img’ you want to modify so you can create a new style for that container.
Here are a few WordPress links to help you edit and modify pages here:
Images: https://codex.wordpress.org/Using_Images
Design & Layout: https://codex.wordpress.org/Blog_Design_and_Layout