there are 2 ways to post a picture.
First, it has to be fairly small.
To resize, check the first post in tech archive/ best of moparts.
Now that you have the right size, you can upload them to a hosting site. That is a whole big tech article, so if you don't know how, google it, or go the other way.
You can post a reply, then check the box at the bottom that says "I want to preview my post and/or attach a file"
this shows what you have written, and there is a box that says "You may attach a file if you like (no bigger than 500000 bytes please)." and has a "browse" box. Click on that and find the picture you want (it helps if you have it on your desktop; easy to find).