Dumb question, though perhaps the article should've said this, but is this unique to Yosemite or are other versions affected? It sounds like it's unique to Yosemite, but further clarification here would be great.
Using Mavericks, I can answer a "yes" to this. It's easy to tell - if you have remote image display off in Mail.app, then you know how a, for example, Target.com or Apple.com e.mail looks with images off. When you search in Spotlight on the Mac, for something like the subject of an e.mail from Target/Apple, it will list the subject, and if you hover your mouse over it, to the left of it, a preview of the e.mail is generated. In this preview, the remote images are displayed.
Something to note - *usually* remote images are used by legit marketers - e.g.: you signed up with Target, perhaps, after you ordered something from them, so you opted into their weekly e.mails. They want to know who's picking up those e.mails and what they are clicking on and what mail clients are being used [which can be useful - that way they make sure to test their HTML e.mails with mail clients that people are using]. If you didn't want this weekly e.mail, you click an 'unsubscribe' link, and it is honored.
A typical spammer gets your address via means that are not by permission and there is no legit unsubscribe link. In fact, sometimes if one is included, it is a baited website that simply confirms you have a valid address and then that address is added to a list of known good addresses and sold to spammers.
What Apple should do is what many other MUAs have done [Mail User Agents]. Make it so if you whitelist an address [eg: adding the Target address to your address book], the remote images will display. But if you get an e.mail from an unknown address, it won't display. Or at least give users that option. I don't mind allowing some legit organizations know that I opened their e.mail and am using a Mac and all that; but I certainly don't want to inform a Russian bride website that the address they sent a message to is valid; they may not take me off of their list, but at least my address won't be added to more lists!
/vjl/