If you want Thrive to fire 3rd-party image or iframe pixels from traffic sources such as Facebook, you must use Thrive's global iframe pixel (not post-back or image pixel) on your conversions.
Traffic sources, such as Facebook, that only provide image or iframe pixels relies on cookies in order to work.
Therefore, when you fire a Thrive conversion, you must use Thrive's iframe pixel so the pixel can be loaded directly in your visitor's browser.
This way, Thrive can also fire the traffic source image or iframe pixel in the visitor's browser, allowing access to the visitor's browser cookies.
If your visitor has cookies turned off, then there would likely be no way to track that visitor at all by the traffic source.
The best way to add the pixel to Thrive would be adding the pixel code to by editing the traffic source, and selecting the "Raw Code" option from the drop down. Then, you can paste all your code: