All the other answers seem to be focusing on the flags in the pictures, themselves, but the question says to look for something *obvious* that is the same  (I believe) throughout all four images.

If you place all images against a black background, you can see what's obviously wrong:

>! [![flags][1]][1]

>! They all have a white border that is not present in the actual countries' flags. Pretty sneaky considering they just look like margins when viewed on a white background, like a PSE question...unless they are meant to be margins and I've got this all wrong.


  [1]: https://i.sstatic.net/fLS3C.png