See explanations
here and
here.
For an image, upload it to an image hosting site (for example
PasteAll), then put the url in the [img]http://www.pasteall.org/pic/show.php?id=
XXXXX[/img] tags. Or, if it is a small file, you could attach it (under additional options)... but keep in mind that forum space is limited, so it may be deleted.