If you see the numbers on the image code (I.e: 700x700) you can actually change that to enlarge or shorten the image to fit all screens. Note that it may need adjusting if you want to change the size and maintain shape.
Typically one would use CSS or even Bootstrap to create responsive images. Because CSS isn’t supported on the forum, it’s rather difficult to make an image that’s responsive to the screen width. I’ve linked a source that may help you, but please keep in mind that there are quite a bit of limitations due to the lack of compatibility between the forum and many programming languages. :)
An image in a topic will automatically resize for the screen. It is a feature of discourse. There is no need to code anything special. I would save the effort of formatting for your VA website so it properly displays across devices.