The best way to store images very much depends on what they look like.
Obviously the most efficient solution is to not need them at all.
So, could they be replaced with primitives?
Or, could you generate them programmatically?
If not, how many images are there?
For one/few image(s) the overhead of a custom packing implementation can be prohibitively expensive.
If however you have many images that have common palettes and/or few colours, SuperPackME or the like can give big savings.
As you should never have more than 1 file inside the jar, are you already combining these 2 png’s into a single combined image?
(and then embedding this combined image inside the class file as a UTF8 encoded String or custom class attribute)