The picture that shows up on posts is an avatar.
Answers to most questions regarding forum operation can be found at the top of this forum under the sticky;
New software features and posting help
Here are the avatar instructions;
To upload or change an avatar:
An avatar is the little picture displayed below your user name in the profile box on the left side of every post. You may use a forums avatar or upload your own custom avatar. Avatars are limited in size to 100 x 100 pixels or 100 KB (whichever is smaller).
To upload a custom avatar:
Choose a picture you like that is stored in your computer. Resize the picture to meet the size requirements above using a photo/image editor or camera software. Images that are too large will fail to load and you will receive and error message.
1. Click user CP (control panel) on upper blue tool bar.
2. Click edit avatar on left side menu.
3. Scroll down to the "Custom Avatar" box and click the "use Custom Avatar" button.
4. Click Browse button. A file menu screen will appear and you can choose the avatar file to be loaded. The file name should appear in the "upload an avatar from your computer box".
5. Click save changes.