Hit your "print screen" button on your keyboard (should be next to F12). Open a graphics editing program, choose "paste" from the clipboard (you may have to start a new drawing with the same pixel dimensions as your monitor display settings first), and resize however you wish.

Make your catsailor post and add the saved image as an attachment during the preview of the post. Once the post is online, go back to the post and view your attachment (in a new window). Copy the URL of the image that just opened in the new window that now resides on catsailor. Go back and edit your post and "insert image" into the text using the URL of the graphic you just uploaded.