upload the image to a free image hosting website: i use TinyPic - Free Image Hosting, Photo Sharing & Video Hosting
click the "choose file" button and find the picture you want to upload.
click the upload button then fill out the 'captcha' box.
select all of the text in the "IMG Code for Forums & Message Boards" box and copy it.
come back to the forum, paste the link you copied and it will look something like this
[I MG] http: //i47. tinypic. com /xsvly.png [/I MG]
without all of the spaces