OK,
first you need to put your photo "on-line" somewhere. This is free is you set up an account at Photobucket.com or similar. You can use several other alternatives but if you use Photobucket this is what you do....
Upload your picture Photobucket - you can follow the instructions on the Photobucket site to do this.
You may want to resize it - you can do that in Photobucket by clicking the "Edit" option above your photo - then choose resize.
See the image below....
I recommend 640x480 as a good image size for the forum (others may disagree !).
Once you have an image the right size, simply copy the text from the box labelled "IMG Code" on the Photobucket page and paste that text into your forum post.
See below...
Make sure you get all the text - including the square brackets at both ends or it wont work. The text will always be more than you can see in the box - you need to scroll sideways to get it all.
That's it.
Hope this helps.
Ian