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:
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.