Once you have the image its a bit like sending an email, the message system , which you have just used to place your question has similar features like add files.

Your screenshot is your file.

Depending on how computer knowledgeable you are there are several options to get the image.

Some computers already have in built programs or you can download them, some of the browsers  have them too, but I cannot explain how your system may do this.

An alternative is a camera, take the shot and transfer it into your computer or  possibly directly from the camera into your message as above.