EDIT: After some mega bumpage, there is now an easier way to post pictures.
1) Click the reply button/new topic in the topic/forum you want to put it in.
2) Underneath the box where you write your reply, click on 'additional options'.
3) You can then browse for the picture on your pc and upload it directly to your post.
Pish.
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Due to the increasingly large numbers of people asking this question, it's easier to write it the once.
1) Save your picture in the .jpg format.
2) Go to
www.photobucket.com and sign up (doesn't take long).
3) Next the upload image box is the browse button, click it and select the file you wish to upload.
4) When the picture is added, underneath you will notice there are 3 boxes with the image location. Copy the bottom one (IMG).
5) Paste the location you copied into your post. Voila, picture posted.