Why would the word "post" be stamped on a radiology image?

Was: Contrast administered? Then "post" might mean it is a post contrast image. It also might mean the type of view that was performed, i.e. a posterior view of a body part. It also might mean a post voiding image in certain types of bladder studies. So it depends.

Posterior: POST is abbreviation for posterior vs anterior projection. This means X-ray beam enters posterior aspect of body and exits anteriorly, referred to as posterior anterior. Anterior posterior is the opposite.

Many possibilites: In addition, "post" could mean following a procedure such as draining fluid from a lung or putting a catheter in a central vein.