Hidden PNG thumbnails, >A FUCKING LEAF's abridged guide:
Prerequisites:
- Ruby
⌙ doubleVision (https://github.com/trishume/doubleVision)
- ImageMagick

I'm assuming you're running $ GAHNOO PLUS LEENOX. If on macOS, use Brew, Macports, or whatever kludge they use this week to get the programs. If on Windows, get Ruby from https://rubyinstaller.org/ and ImageMagick from https://www.imagemagick.org/script/download.php#windows
This should go without saying, but CLI knowledge is required. Adapt as necessary for Windows.

Step 1: Prepare your images
Once Ruby, doubleVision and ImageMagick are installed, you'll need two images: one the 'real' image, and the second your 'fake thumbnail' image.
First, ensure the images are of a similar aspect ratio. We'll be resizing both images to the exact same lower resolution, so this is needed to ensure not too much distortion comes through.
Pics #1 and #2 are my prepared examples for demonstration purposes.
Our first command creates our hidden 'secret' image with a a limit of 1000 pixels in either dimension, removing all transparency and replacing it with white:
$ convert ./input-secret.png -resize 1000x1000 -background white -flatten ./secret.png
Make note of the dimensions of your created image, in this instance 704x1000. For our thumbnail disguise 'mask' image, we need to resize this to perfectly match:
$ convert ./input-mask.png -resize 704x1000\! -background white -flatten ./mask.png
Do not omit the '\!' characters after the -resize 704x1000\! flag.
If all goes well, you should have two identically sized images.
Adjust parameters to taste.

Step 2: Merge the images
Merge the two images with the doubleVision command:
$ doubleVision ./degen-secret.png ./degen-mask.png ./degen-result.png
You should create a PNG file that will contain both images. Opening it in common image viewers will show you the 'thumbnail' version, but right-click and open it with a browser, and you'll see the real image concealed underneath.
See Pic #3 for my example's final result.

Step 3: Raid and Pillage
Build your collection of images, and let loose the triggers of war.

This trick works because of 4th-dimensional Warp fuckery with PNG gamma settings. If you care for details, you can read further at http://thume.ca/projects/2012/11/14/magic-png-files/

Anyways, I'm going to bed now. I expect a thread full of disguised shock images when I get back. Don't disappoint me, autists.