1 - Host image on a image hosting website (eg photobucket.com)
2 - Upload image a get the url for the image (Photobucket, for example has a feature that allows you to simply click a box that already has the forum tags included, copying everything you need to your clipboard). If this feature is not available simply right-click on the uploaded image, go to properties and copy the
http:// address.
3 - Paste image url into your forum and wrap the url in these tags
(with spaces removed)
Result:
Done.