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Hidden images (e.g., among others, the cannabis plants...) sometimes are hidden just for fun.

From severnbeachantiques.com/famous-rare-1980-huntley-and-palmer-rude-garden-party-ginger-nuts-tin you learn about one among many reasons why artist hide images in images:
"I did them out of devilment, purely for a laugh. I've always been a bit of a naughty boy but I've nothing against Huntley & Palmers. There have been rumours that I got made redundant and did it out of revenge. But that's not true - I was only ever a freelance. I just felt like adding a bit of smut to the proceedings."
That is what Artist Mick Hill said about his hidden surprises in the cover (1980) of a Huntley and Palmer garden party ginger nuts tin.