You can't. In the current version, this is not changeable. Your request is
evidently popular, so a future version might let you set the option to
download them automatically.
Consider this, though. If you hate spam, then clicking on the "Download
images" link is a small price to pay. Why? Spammers get more money when
their emails are sent to legitimate addresses. So if you automatically
download images, in some cases a reply is automatically sent back to the
spammer. This email verifies that the email address they used was
legitimate. What's the result? Those spammers can sell your (verified
legitimate) address to other spammers for higher amounts than unverified
addresses. In other words, more profit to them. And more crap in your
mailbox.
If you suspect that the item is spam, it's easier just to delete it without
clicking on the Download Pictures link. This spares verification of your
address to the sender, and saves you potentially tons of spam.
Even if MS includes automatic download of pictures in a future revision, I
would caution you not to use it.
On the other hand, if you love spam, click away!