I say "really old" because at least in the 17th century, they are all photo-realistic. (I still don't understand how anyone can paint that beautifully and realistically.)
However, let's say the 15h century or earlier, all the paintings seem to use a completely warped perspective. Nothing seems to "make sense" proportionally; people and objects seem to be able to have any size and proportions that the artist feels like using at the moment, or found convenient, or whatever the reason may be. A large ship can be represented by a contraption that barely fits a couple of people side by side, with humans of the same size next to it. It looks very strange to me, every time.
It's almost as if the entire concept of "perspective" was invented just a few hundred years ago, but that sounds bizarre. Us humans have had eyes for an extremely long time, and our brains surely haven't fundamentally changed in that short period of time, so we clearly knew that a large ship is... larger than people. It seems to make no sense to paint in such a manner. It couldn't be that they didn't "understand" this, or that their skilled fingers somehow couldn't paint it onto a canvas.
They must have had some reason. I once heard or read somewhere something about them considering the size of an object or person to equal their or its importance. While strange, it could somewhat explain this for me, but I still think it sounds very odd that they would not simply make the king or queen "glow" and have a central position in the painting, perhaps standing close to the "camera". The wildly varying proportions of everything makes it seem very abstract and surreal rather than "real".
Those beautiful, realistic paintings just a couple of hundred years later seem basically like (very well shot and lit) photographs, and I can stare at them forever in awe. The ones with "random proportions" make me angry and confused more than amazed most of the time... although the fact that they really did this is in itself fascinating to me. Which is why I ask.