The image must be hosted someplace and you must have the URL for it. You can host images on websites like Photobucket, Flickr, Picasa, etc. or in your Image Locker on The Duck.
Once you have the URL, use the BBCode [img][/img] tags, or with the Insert Image button on the "What you see is what you get" (WYSIWYG) visual text editor available in some places, like the forums.
[img]url of image[/img]
For WYSIWYG editors:
Note that you do not need to fill in any of the WYSIWYG's fields other than URL if you don't want to.
In places like the forums,
any images you post which are wider than the postable area, the image will be
automatically restricted in size to fit inside the area.