Photos of smiling corpses is one of the popular propaganda material used by Pseudo-Jihadi Salafis. In their view, it is one of their strongest evidence they possess to convince themselves that they are on the right path and soon to be green birds of paradise.
Even though of course smiling corpses can be found in plenty among people of all religion and that the photos used by pseudo-Jihadi Salafis are very selective and reported by their own unreliable propaganda team, at the end of the day the whole basis of smiling corpse as evidence is not known to Muslims. We have not heard of anyone from the Salaf or Khalaf looking for smiling corpses as evidence nor found anyone to have interpreted any verse from the Quran or saying from the Prophet in such a manner.
We also, on the contrary, find from the teachings of our beloved Prophet that the sign of the death of a believer is that the believer dies with sweat on his forehead, as can be seen from the authentic hadiths below and which is also consistent with how our beloved Prophet passed away.