Images excel at conveying aspirational and lifestyle messaging. That's why high-end and experiential brands like Neiman Marcus and Sephora compose their emails mostly of images. To be convinced to buy a $1,000 dress or a shade of lipstick, consumers have to be able to see It -- and typically they'll enable images to see it.
On the other hand, text connotes value. It's the secret language of sales. That's why value-oriented brands like Wal-Mart and Overstock.com use a healthy amount of text -- including lots of HTML text -- and why higher-end brands use text to announce sales.
These two emails from Ralph Lauren are a great example of the different approaches: This email is aspirational and this one is value-oriented. Lots of retailers exhibit this design behavior, so over time we've trained subscribers to associate text with sales.
The Growing Influence of Text for Sales
Some retailers have been elevating that discount messaging into parts of the email that images don't typically or can't go to make that call-to-action more prominent. For instance, over the past couple of months nearly 8% of top online retailers have included a discount code in their preheader text, making it the first thing subscribers see when they open up the email -- and allowing them to take action on the sale without reading further. EBags used this tactic recently in this email. When the email is principally about the sale, you really don't care if they read the rest of the email, so long as they act on the sale.
A few retailers have elevated their sale call-to-action all the way to the subject line, allowing subscribers to act on the discount code without ever opening the email. JCPenney and RitzCamera both recently used this tactic:
JCPenney - Use Code 32GIFT to Get Free Shipping!
RitzCamera - Super Student Special, 10% OFF (Use Coupon Code 1828)
Again, if your goal is to drive action, why not lower as many barriers to action as possible? I'd be curious to see if emails with subject lines that include discount codes are shared more than those without.
In the age of Twitter, I expect that we'll see more subject lines that allow subscribers to take action without having to open the email. At the same time I'd conjecture that this approach might actually lead to more opens because subscribers will have already been engaged by the subject line.
So be wise in your use of text and images. Don't shy away from images -- even really large ones -- if your brand is aspirational or the circumstances call for them.