How can I move junk emails on my iPad without displaying the message body?
A spammer could validate reception by inserting an image which is hosted on a remote server in a HTML email. A unique code per recipient could be appended to the URL of the image, at which point the remote server can know which addresses recieved and opened the email.

While the default setting on iOS is to load remote images automatically (and hence makes your email address vulnerable to this kind of tracking), you can change this to Off in Settings - Mail, Contacts, Calendars - Load Remote Images.

Please note that this will prevent any remote images in any email to be loaded automatically, and therefor possibly breaking the layout of HTML emails, but you can always manually load the images in an email by tapping the appropriate button at the bottom of the email body.