When you goto Post Reply there should be a postcard looking thing and click on it and put the URL to the image in the box that pops up and it will work if not use this:
generally it's frowned upon to use images hosted somewhere else and eat up their bandwidth so a good site to use is photobucket where you can upload all your images at desired sizes and have them host it for you and they will provide a pastable IMG code as well as other pastable codes
In many cases websites take down or change the address to images that cause a lot of traffic such as new video cards, and if you host it yourself this is one thing you don't need to worry about