In C-LINE, the points are the receptor locations where the concentrations are modeled. These concentrations are used to generate the overlay image, with different approaches used for short-term vs. long-term (annual) results.
When viewing the results from computing short-term concentrations, each “block” in the overlay image would be centered around a receptor (the exact number of pixels in each “block” varies depending on the modeling domain). For long-term concentrations, the receptor concentrations are interpolated, so the receptor location doesn’t correspond to particular pixels.