You need to upload it to somewhere. Google: timy url. don't know the exact name of the site but it's along them lines. You upload a pic on that site your given a url for the pic so you just copy n paste the url into your comment or whatever your posting
(of course, the picture has to be on the internet before that... otherwise it won't have a link, hence it can't be posted here)
If you're wanting to include pictures from the internet, just copy and paste the pictures URL address and we'll embed it for you after you've submitted it. Keep in mind, embedding does NOT happen in comments or question titles, just question details and opinions. If you want to upload your own to share, you must upload to some other site like imgur, photobucket, tinypic, etc. After you've uploaded there, again, copy and paste the link and it'll embed for you.
This information can always be found in our FAQ, please check it out :-)