Is there a way to block images from untrusted senders on the iPhone?
Doubtful without a custom email app of some kind.

Certainly the default iPhone mail app won't selectively (or conditionally on click) block images by default.

Have you tried the official GMail iPhone webapp?

http://gmailblog.blogspot.com/2008/01/new-gmail-for-iphone.html
http://www.google.com/mobile/mail/

Not a native app, but... there's surprisingly almost zero native iPhone apps that do Gmail.