The size of decoder is so small, so one can append the source code of decoder to most images and not make them much bigger in the process.
But what if the decoder rots?
You mean standalone C99 code becomes non-compilable? Absurd.
The author notes his decoder is 100 lines of libc-free C code: these files are safe!
Why wouldn't they just use TGA? Just as simple, more flexible if need be and understood even today by most image manipulation programs and 3d modelling software.
I spend some free time reverse-engineering obscure game formats, and my observations is that this is universally true of everything.