There are 2 methods, add them as an attachment, or link them from another website.
Method 1 - Upload them here:
When creating or replying there is a section below the text box called "Upload Attachment"
Click the "Choose File" button, which pops up a file browser to find the file on your computer.
Select the file and click open.
Id you want you can add a comment the the file comment box.
then click the "Add the File" button.
Repeat the process for each image/file.
Then click "Submit" (above the Upload attachment heading) to post your message with the attached files.
Note: If your images are too big the upload will fail (We dont want to fill the server up with 5meg image files when a 1024x786 image will do just fine).
It would be nice if you could resize the images before uploading them.
If you are using Windows install this:
http://imageresizer.codeplex.com/
This will give you the ability to right click the image and resize it easily before uploading.
Method 2: Link them from another site.
If you have a flicker/photobucket/etc account you copy in the links by clicking the "Img" button above the message box and paste in the url to the image between the [ img ] [ /img ] tags.