Depending on the picture that your looking to post there are a few ways to post pictures on to the site.
1. If you are posting a picture say from RCDB.COM, first youll need to right click on the picture to get the properties of the pic.
http://cache.rcdb.com/pictures/picmax/p28422.jpg Now if you just copied and pasted that link into your address bar, you'd get only the picture you see.
But if you use [img]website%20info[/img] you'd get the picture onto the thread here.
Trying it out, make sure you put the link in between the brackets.
This is where the preview tab will come in handy, I always do a quick preview before I post, to make sure i didnt mistype or anything like that. And as you can see the picture is there!
Now, if you want to post a picture that YOU took, say it was an awesome picture you took of Raging Bull or you went to the park and want to do a Photo Trip Report, you will need to use a source like photobucket.com to host the picture first, and then be able to post the picture here.
Hopefully this helps, if you need more help feel free to message me via AIM, PM or FB.