My understanding is that if you are uploading images (i.e., attaching) you can only add one to a post. If you are linking to an image (i.e, using the IMG code from the little picture icon above), you can link to multiple images on the web from within a post. Although with the latter method, there's something not "polite" about it I believe. Something to do with using the other site's bandwith or something. Pauline, Marta or Chris know better than I.
Yes, that is right. You can only upload one image but you can link to images on other web pages. You can link to the photos using either the img menu item (the little picture button) or by directly embedding the html code.
When you link to another image, you are using bandwidth from the other website so I would only recommend linking to images on a personal website, Slow Photos or a public photo sharing site such as Flickr. Flickr is easy because it will give you html code to link directly to the different sizes of the picture. I think an upcoming version of the software for Slow Photos will also have a similar feature if we enable it.