1) To see the texture:
Go to the torishop > left side > click on "deactivated" > find the item > click on "activate" next to the item name > go in-game and type /dl <your username here>
Make sure you have the latest version and that you have body textures enabled through in-game....
Game menu > setup > options > advanced > check the box next to body textures if it isn't already checked.
2) The difference: its really just the pixel size and quality of the images that are put on the textures. The images will look better with the more pixel space allowed. Hence why 512 is supposed to be more expensive and more popular than 256 or 128. If you make textures, you need to make the texture on a canvas that suits the size... 512x512 for 512 textures, 256x256 for 256 textures, etc.