We just helped a MailChimp customer with a little bit of a deliverability problem. He’s been sending campaigns without incident for years now (since July of 2004). He’d send a few thousand emails to his list, and get a 3% bounce rate. Pretty normal for a well-maintained list.
Recently, he sent a campaign that was nothing but one big graphic, with a little line of unsubscribe text underneath it. That campaign got a 26% bounceback rate. Ouch.
The reason? AOL blocked his campaign.
Today, he switched to a more traditional newsletter layout (with a better balance of text and graphics) and got a 2.2% bounce rate this time around. Much better.
As more and more spammers try that "image only" tactic, spam filters will adapt and begin blocking legitimate marketers who send image-only campaigns. Be especially on the lookout when you’re sending event invitations, because it seems most marketers make those one big image, with no text to balance it out.