Pictures that invoke so much adoration that they cause near physical harm to occur.
This is far above the level of just cute or even adorable. Pictures that are HHHNNNGGGGG worthy.
When in doubt, observe the first row of images on the first page. Also, do not automatically add any pic of Flandre Scarlet or Sukuna Shinmyoumaru that looks cute to this pool. Same goes for any picture that merely has cat ears and/or chibi. Expect the pool to be occasionally culled of images that do not fit the criteria.