When you click the "post reply" button, it'll go into a new page where you'll see a big text box/smilie icons/etc, below that will be "addtional options" and you'll see a button that says "manage attachments".
Click "manage attachments" and a new window will open and it will allow you to choose each pic you want to upload. After you have chosen each pic, click the "upload" button in that same window and allow them to upload.
After they have uploaded scroll to the bottom of that window and click the "close window" button. That window will then close and you'll be back to your main MFK window. Then hit "submit reply".