Method One - Link to Offsite Image
First of all you need to have uploaded your images to a web server. Many ISPs offer their users a small amount of webspace which could be used to store your images. Alternatively, and probably more conveniently, there are many free images hosting services out there that make life a great deal easier. See this FAQ for a list of possibilities.
Once your image is uploaded, getting it to show up in your forum messages is simply a case of enclosing the url for the image within the IMG tags as shown below:

Method Two - Upload an Image as an Attachment
We now allow you to upload images (and other files) to our servers as attachments. To do so, first click the 'Upload attachment' tab and then browse to the image file on your PC you wish to upload:

You then have to click the 'Add the file' button and wait a while for the image to upload to our servers. If you then click the 'Place inline' button, the appropriate link to your image will be automatically added to your post at the cursor location:

Now, when you post your thread, your image should be displayed as an inline thumbnail of the original. Clicking the image will show it at full size...try it out on the bream below!!