Although I do not have any proof that the creatures imaged below are aliens, or that they are even real pictures they certainly look alien to me and many of them have a strange inteligence in their eyes that tells me that they knew just a little more than they are letting on. And someday when we do run into aliens i think they might just look like one of these creatures.
While neither an animal nor an alien the image below is one of the less disturbing of the images. Made of vegitables this alien is also the most likely thing on this list I woul eat.
There is no real reason to assume that the human customs would mean anythnig to an alien species. For example this species shows none of the modesty compulsions of the modern human.
Perhaps what is most disturbing about this alien creature is that it almost looks like the features match but they do not quite look right. The eyes are it seems not eyes at all. Perhaps some sort of ear or some sense organ we do not even yet understand.
This is the alien we are most likely to have trouble with in my imagination. Almost human looking the strange eyes are almost an insult and make everyone who sees them squirm.
Finally we have the most intelligent of the aliens this creature is also clearly evil. You can see the venom in the eyes and the scowl is unmistakable . This alien cat would be just as happy to kill you as to treat you as an equal.