There are two ways. Either when making the post, below the textbox and everything there is more options. You can attach them there by clicking the button saying "Manage Attachments".
Or you can upload them to the United Bimmer File Manager. Click here
to go there, and just browse for the file, and click upload. After it's done, copy and paste the location string it tells you into this of a post: [img]XXX[/img] where XXX is the location string.
That make sense, or do you need me to clearify any part?