The best format is JPG. You can also use BMP, TIF, GIF and others, but JPG takes advantage of the auto-thumbnailing built into the forum software.
The size should be no greater than 800 x 800 pixels.
The color mode should be RGB! CMYK files will generate errors.
Uploading (attaching) is done by posting a reply to a post or starting a new thread and then scrolling down to the blue area beneath the reply box. There you will find a button called "Manage Attachments". Clicking it will bring up a dialog to find and upload the image from your hard drive. Once uploaded, complete your post and click on the "Submit Reply" button.