Well, the server performance doesn't suffer very much from posters. Only thing you have to keep in mind is the used bandwith when a client downloads an image. A client only downloads images which are in his proximity, and only if there isn't already a cached version of the image in his game directory.
However, when it comes to the clientside performance, things are getting more critical: Basically every "visible" image has to be loaded into the player's RAM (direct memory) and VRAM. If there are too many different images close to each other, some players (who don't have that much RAM) may run out of memory. Here it's important to limit the maximum image size per user (you can do that in the permissions). You have to keep in mind that a 1024x1024 px image consumes 4 times more memory than a 512x512 image, for example. So having many small images isn't a big deal (e.g. having 1000 32x32 px images consumes less memory than a single 1024x1024 px image), but it becomes a problem when using many big images.
To cut a long story short: Don't worry too much about posters, it really depends on where those images were placed (if a player has a lonely house far away from everything else it's really not a problem when he uses many images), and also the max image size matters.
Btw, the file size on your harddrive does only matter when it comes to the bandwith. When loading an image into memory, we can't use compression yet (due to patent reasons), so the memory consumption per images is much higher than the actual file size (it's basically like storing a .bmp image).