Basically there are two things to think about - first is actually putting the image online somewhere, second is making the code within your post.
If you already have the image at some URL, you can simply put it in your post by clicking on the little yellow "Insert Image" icon above your post and entering in the URL.
If you don't have it online already, you can upload pictures at any number of websites. www.imageshack.us is a good example. Once you upload your image it will give you some code you can simply copy-paste into your post to bring the picture up. The code will start with square brackets with the word "img" inside them.
While you get comfortable with it preview your posts instead of posting them to see if the image appears. If it doesn't appear in the preview, it hasn't worked :)