You would probably want to use [%txn_id%] for the unique Transaction ID generated by the payment processor for each completed payment. Other template variables you can use in your E-junkie thank-you page are documented here:
http://www.e-junkie.com/ej/help.custom.thankyou-page.htm
However, if you're intending to place this tracking pixel code in your E-junkie thank-you page, bear in mind that we cannot control checkout behaviors on a third-party payment processor's site such as paypal.com, so we are unable to force them to auto-redirect buyers to any thank-you page after they complete checkout. If you aren't selling downloads (in which case the buyer has an incentive to visit their thank-you page at some point, where they would claim their download), you may not be able to track conversions accurately via thank-you page visits. The top of this page explains which payment methods can and cannot auto-redirect buyers to a thank-you page:
http://www.e-junkie.com/ej/help.tracking.htm
Have you considered just using our own, built-in affiliate system? That tracks all affiliate conversions and commissions quite accurately via our back-end order-handling system without depending on unreliable thank-you page visits:
http://www.e-junkie.com/ej/help.affiliates.htm