Im not sure if it works the same on this site as i am yet to post one, but firstly you will have to set up an account on Image hosting, free photo sharing & video sharing at Photobucket

On there you have to upload your photo's and once this is done, when you look at your photos, it will give you a URL code. If you copy that URL code, click the Insert image option when you are typing a reply (5th image from the right, looks like there are small mountains on it lol) then paste the URL in there and click OK and it should insert your picture

Sorry its not very clear, but if you do have any problems, please let me know, will do my best to help.