What you're looking for is probably something called Stenography and/or alternatively Obfuscation Is that correct? If yes;

then the answer to your question "Is this a good idea" would be, yes and no. It depends how you use Steganography or Obfuscation.

To give a really obvious example of the answer no: If you tell someone to find a hidden message inside of the image.

But what if you don't give any clues, and i.e placing the picture in a folder with lot of other pictures that looks kinda the same(like, same colors, and sizes, but different texts)? Then, it would be more secure. And, "a good idea" is maybe is a bit broad, do you mean cryptographicly secure? that it would be distinguishable?

What you're looking for is probably something called Stenography and/or alternatively Obfuscation Is that correct? If yes;

then the answer to your question "Is this a good idea" would be, yes and no. It depends how you use Steganography or Obfuscation.

To give a really obvious example of the answer no: If you tell someone to find a hidden message inside of the image.

But what if you don't give any clues, and i.e placing the picture in a folder with lot of other pictures that looks kinda the same(like, same colors, and sizes, but different texts)? Then, it would be more secure. And, "a good idea" is maybe is a bit broad, do you mean cryptographicly secure? that it would be indistinguishable from a normal image?