The tracking pixel image is a way for both you and the merchant to monitor impressions and clickthrough rate. It's one more way to track performance. Sales would still be tracked if you eliminated it, but useful information would be lost.
The <img> bit does not need to be "right next door" to the main tracking link. It could be somewhere else in the page code and still do its job.
If you move tracking pixels around, be sure to remember what's what so that if you change any links later you don't end up with stray tracking pixels that no longer match the main tracking link.