Ok, it looks like you had the picture address a little off, sometimes photo hosts will give you code for different applications, most post boards use UBB code that recognises a URL, this is what you posted;
[img]http://yfrog.com/5bruger7744groups1p[/img]
This is what the URL address is;
http://img191.yfrog.com/img191/7673/ruger7744groups1.pngSo add the image tags and it will display the picture
[image] URL address here [/image]
Or as you can see that the address is highlighted in blue, I didn't do that. It's because this board recognizes the URL and automaticly makes a link to the picture. Just click the blue "link" and it opens a new window with the picture. The tags just take the next step and display the picture in this thread.
Another way to get a URL address for a picture is to put your cursor over the picture and right click. A window will pop up aksing what you would like to do, save the pic, e-mail...ect. At the bottom of the list there will be properties, click that and another window will pop up. This window gives you the info on the picture. Find the URL address and copy it. Then just past the address in the image tags, or image box from the menu at the top of the post box.