If the picture is on a web server
(such as might be found via the Google Image Search):
To add an image to your post, the bb code is:
(where the *** is replaced with IMG)
which produces the following image:
If the picture is on your personal computer:
The image can be uploaded as an attachment to a post.
Below the post creation/editing box, you will find "Additional Options."
In the "Attach Files" group, click the "Manage Attachments" button.
A pop-up window allows you to specify the file on your computer that you want to upload.
After specifying a path to your image file, click the "Upload" button.
A thumbnail of your attached image will then be automatically generated for display in your post.
See below for a couple examples: