Image Tracker as TrackingPixel

Hi, I’m developing a portal where I’ve integrated piwik by JavaScript.
When a visitor places an order at our partner-websites we would like to have the possibility to embed a TrackingPixel that identifies the same user by reading out cookies. The TrackingPixel-URL should be always the same without any dynamic visitorId or something similar.

Is this generally possible or do we need to add a visitor-identifier to map this visitor’s action to the same session?
In the docs i found this statement: “The image tracker code also does not create first party tracking cookies which results”. In my scenario a cookie had been already created during the initial JavaScript tracking within my portal.

Thanks in advance