Assuming you are not talking about
steganography, you can hide
a big image in a bigger file (perhaps the opposite of your question),
simply by concatenating it on the end of another smaller image.
Though you wont fool
ls you will fool many programs that display or
manipulate the image, as they will only see the first part of the file and
report the size of the first image and ignore the second bigger image.
cat a.png b.png >new.png
identify a.png b.png new.png
a.png PNG 114x98 114x98+0+0 8-bit sRGB 2.51KB 0.000u 0:00.000
b.png PNG 81x54 81x54+0+0 8-bit sRGB 256c 2.86KB 0.000u 0:00.000
new.png PNG 114x98 114x98+0+0 8-bit sRGB 5.36KB 0.000u 0:00.000