Sorry to bring this thread alive after so long but did a search as I was going to ask the same question. I've recently switched to AquaMail from TypeApp and I love everything about AquaMail! Only thing I miss is TypeApp's ability to display contact images for senders not in my address book.
Kostya, Besides the fact that it's been years since this initial post (should ease your concerns) there are ways to do this right to avoid any legal worries. I see two options
1. Ideally, you could simply look up favicons from the senders site and cache the images. This wouldn't be a legal issue as the images is being used for it's intended purpose, that's what browsers do when you bookmark sites. Google even offers it in their search functionality
http://w w w . g o o g l e . c o m/s2/favicons?domain=w w w . a q u a m a i l . c o m
Besides it simply looking cool, one added benefit is the ability to easily identify phishing emails that pass through email filters through contact images. Example, if I get an email from paypal-accounts@bobshotsouce.c o m, I'm going to see a different contact image before I even open the email. I'm sure PayPal would appreciate that
2. Not as ideal but you could have a directory where users could dump their own images to override contact images. ie if an image called paypal.com.png exists in that directory, use that.