I think from your own PC you can't. What you can do though is click on what you have posted and a box with a larger image shows up and if you click on that again, a much larger image shows up.
From an external hosting site you can post larger images, the forum software still scales it down though to suit a page size for here. So from a hosting site I use image tags and that shows a larger image. Some here link the image and image tags from their flickr account. I'm not sure exactly how they do that though. Directly uploaded from your PC will always show a smaller image to start with as you have there.
All the best and sounds more complicated than it is from a hosting site.
Danny.