Sure. Since yours is on a computer I will start with that one.
You need to use a separate site to upload your picture to - I like to use tinypic.com . Another good one is photobucket.com . You can upload pictures without creating an account on these websites, but I have an account so I can remove or find my pictures again without trouble. A third option is to upload the picture to your facebook (if you use this option you'll have to follow the set of instructions for an image already uploaded somewhere).
Here is a shot of the upload screen from tinypic.com:

Once uploaded, the website you use will provide the code you need, example shown here:

Copy this code, and you can post it in the forum - just make sure if the code has capital IMG to change it to lowercase or it might not show up properly.
Now if the image is already uploaded somewhere, you need to right-click over the image and left-click on "copy image address".

Then come to the forum, and when making a post click on the "Img" button. You'll paste the copied image link in-between the two img.
EG, Img button by itself:
Img code with image address:
Code: Select all
[img]http://signavatar.com/39720_s.gif[/img]