If you mean that you have produced a screen-cap, so long as it is a JPEG, or PNG image, you can attach that to a Reply in the forum, by going to the little "camera" icon in the middle of the Toolbar, and then navigating to where you have the JPEG, or PNG, stored on your computer. That will add the image to the Reply, like this:
Note: that Toolbar is from what is now the Advanced Editor. in the regular Editor, there will be fewer icons, and their arrangement will be slightly different, but the icon for Attaching an image is the same.
Good luck, and hope that helps,