We want to advertise our discourse forum on Google Adwords and Facebook Ads. They allow us to place an image pixel when a conversion takes place (in this case a user registration). This allows them to optimize our ad campaigns and show our ads to users who are more likely to signup. Does that make sense?
Here is more information on how to use image pixels for conversion tracking:
https://support.google.com/adwords/answer/6331314?hl=en&ref_topic=3165803