The first step is to upload your image to a image hosting website like
Photobucket or
Imageshack. Photobucket requires for you to make an account but I believe that Imageshack allows you to upload without registering.
What you'll need to do is browse for the file and then use the upload.
The next step is to get the static url or web address of your image and either use the IMG tag button in the posting screen or use this code.
(IMG)image_address_here(/IMG)
replacing the parentheses () with brackets []
Hope this helps.