1. Hit the "Post Reply" button, or click on "Go Advanced" in the Quick Reply box
2. Hit the "Manage Attachments" button under "Additional Options"
3. Upload your files, one at a time. You can upload as many files as you want. Yes, more than one.
4. Write something
5. If you want to control where an attachment is placed in the post then click on the paperclip. You'll be presented with a list of uploaded files. The attached file you select will be inserted at exactly the position the cursor is at at that time. If it's an image, it'll get displayed.
Even if it's a BMP image, I checked!
Unused (not inserted) attachments will be appended to the end of the post.
I think that's all there is to know. A collage where all the stuff is shown together in the first post would make sense, too.