After you type your text in a post, make sure the box "I want to preview my post and/or attach a file" is checked, and then click "continue" at the bottom of the dialog box. On the Preview Post page, after the comment "You may attach a file if you like (no bigger than 100000 bytes please), click on the "browse" button. Browse to the floppy disk on your computer, and highlight the file you wish to attach. Then click "Post This Message"
The attached file must be under 100,000 bytes in size, and convention is the maximal size is 800 x 600. *.jpg files will give the best pictures within these size limits.