That's a great rule for general purposes, but it's intended for situations where you have lots of small assets that you are asking the client to download.
In this instance we're talking about 2 megapixel images, so the extra HTTP requests are not even going to register in the mix. With modern browsers pulling down 6+ parallel downloads, breaking the image up might make plenty of sense.
If you use domain sharding you can get more parellel downloads on older browsers which reduces the problems some due to more requests (parellel request limit on old browsers and unneeded cookies for static resources, but theres still overhead), but that's a separate issue. It's best to avoid the problem altogether by minimizing HTTP Requests (as you suggest).
http://developer.yahoo.com/performance/rules.html