Attachments are good if you have a pic that you want to show in one single post. If you have a pic that you will be putting into more than one post, then you can add it to your uploaded files, which is a little bit different way of doing it. For that you go to "User CP" and use the "Upload files" link on the left menu. Then when you want to use one of your uploaded images. You click the "My Images" button that is there when you are making a new post.
Also, if the picture is already hosted on the web somehwere, you can just copy the web address, and put it between img tags like this: [url]http://www.tacala.com/images/soft-taco.jpg[/url][color=#000000][/img[/color]]
A good site for hosting pictures is
[url="http://www.imageshack.us/"]http://www.imageshack.us/[/url]
It's all free, and pretty self explanatory to use.