This notion of hidden images in WTS publications, and the reported exposure of them, goes back a while, to the 1980s. There was gossip then among R&F JWs about hidden or subliminal images in the artwork. In the late 80s an active Witness named Darek Barefoot (yes, first name spelled correctly) attempted to expose this matter secretly within the org, knowing he'd be shut down if he tried openly. He was from a respected multi-generation JW family. Eventually he was exposed by a fellow Witness coworker and his entire family was DFed, including members who had nothing to do with his attempts at exposure, for not reporting him and denouncing him.
His story is published in his book JWs and the Hour of Darkness, 1992 red cover, revised ed. green cover, later the same year, now both out of print. There are plenty of examples of the images in his book. The issue of hidden images in the WTS artwork has been brought up in various posts on this forum from time to time since, with mainly reactions of "I can't see it," "this is hogwash," etc. though some here agree that the images are present.
If they really are there, then the reasons behind them and the org's denial and defense of the people responsible for them becomes the resulting issue. Were the pics created by in-house artists and sneaked past blind overseers? Did the artists have the goods somehow on the Bethel heavies, so they were tolerated out of fear? On and on such conjectures can go.
You can search yourself for these past posts on this site's search function. I don't know whether anyone has scanned Barefoot's book and made it available online or not.
Hope this helps with some history.