Let's see what's going wrong here.
Do you have a home for your image, such as Flickr, Picassa, or some other site?
Do you have the exact url of the image, such as http:// whereitisstored.com/graphicname.jpg? I know in Flickr it's a bit tricky - you can't just right-click and get the url. For example, here's a photo of one of Jan Tz's quickbreads from my Flickr stream. I have to click on the photo, click the box with the arrow (lower right hand side) to open "other ways to share" and a box pops up with various ways to share, either by direct link or Grab the HTML/BBCode. These forums use the BBC code so I copy and past all the stuff in that box. Here's a photo of the box:
Using that code, I can post this photo:
If you're posting a graphic found on another web site, like this cover of the Starch Solution on on Amazon, or one that doesn't already have the BBCode:
just copy and past the image url (I'll leave spaces in my example so it doesn't show the image, just the words)
http:// ecx.images-amazon.com/images/I/51NdE1gj2TL._SY344_PJlook-inside-v2,TopRight,1,0_SH20_BO1,204,203,200_. jpg
Sometimes all you have to do is post the image link and the board software automatically formats it as a graphic, but I usually play it safe and highlight the entire link and click the "IMG" button at the top of the edit window.
If you have any more problems, just holler.