Black Bird Image Optimizer Pro works by reducing [discarding] some of the data stored in an existing image file, making the new file smaller -- how much smaller depends on the image and how much color data is stored in that image. An image without a lot of color that had filters or FX applied to smooth [blur] the textures [looking almost more like an illustration] had its file size almost cut in half. With a quick glance it appeared unchanged, though I'm sure you could find changes under close inspection. A noisy photo with Lots of color data had its file size reduced to less than a quarter of the original size, but colors changed very noticeably. Both were jpg format, and it does not look like the app simply decreased the jpg quality level. The closest you get to an explanation is the notation in the right hand image panel saying that it tries to find the best png [file format] colortype, which basically means getting rid of some of the data stored for each pixel.
w3[.]org/TR/PNG-Chunks.html
A little background...
Every image or photo you see on a display -- a PC monitor, laptop screen, cell phone etc. -- is made up of pixels, which can be thought of as the smallest dot of color displayed. When a Raster [pixel-based] image is stored in a file, data is stored for each pixel, with the Bit Depth = the number of bits stored per pixel -- obviously the more data stored per pixel, the larger the file, so a 32 bit image is larger than a 16 bit image is larger than an 8 bit image and so on. Tossing out some of the bits per pixel, &/or reducing the amount of colors available [the palette] makes files smaller. Additionally, different image file formats have different ways to reduce the amount of data that's stored, e.g., the jpg quality setting [changing the quantization], making file sizes smaller.
Black Bird Image Optimizer Pro is a small app [~3MB] dated April, 2018, understandably using some older file versions -- you can update two of the files yourself.
pngquant.exe -> pngquant[.]org/
jpegtran.exe -> jpegclub[.]org/jpegtran/
Installing Black Bird Image Optimizer Pro adds the program's folder plus a new folder in Users\ [UserName]\ AppData\ Local\. I recorded 227927 registry entries added in my Win11 VM -- I Very obviously might have missed something, but I *think* the only registry change was the addition of an uninstall key, the rest being a component hive rewrite. I copied the app from my VM & activated / ran it in Win11 just fine. In the comments on the GOTD download page someone said something about the app uploading files online, which I guess might be understandable since the button to load an image says Upload. For a simple test I unplugged my VM, running the software with no network connection, and it did just fine.