I'll guide you through the steps that I've taken myself in adding my own images to the forum. I understand that not everyone will have the same software as me but hopefully you will get the gist of it.
First off you cant upload images to the forum so you have to host them yourself on your own web space (hopefully you know how to do this) If you dont have web space then I can host a photo for you if you send it via email.
Step 1
Resize your image, 200x200 pixels is more than enough. I use adobe photoshop for all my photos, it will allow you to open an image then resize it to 200x200 pixels then when you save it you can set the quality of the image to lower the file size (the lower the better).
Step 2
Reply to the members post and leave a message or whatever then click 'Insert Image' button at the top of the page above the smiley faces.
This should appear
{img] [/img]Insert the url of the image you wish to add between the brackets, for eg.
{img]http://www.myphoto.co.u/myimage.jpg[/img]Step 3
Click post and you should be done.