I'm building a game which uses isometric tiles as terrain, and I'm seeing some really odd visual glitches... Almost like it's anti-aliasing the images. Here is an example of one of the tiles I'm using:

Pixel-wise, it should tile perfectly, with no borders visible. However, for some reason, when I display it in-game, I get this effect:

(taken on Windows)
I'm fairly confident that this isn't part of the png image itself. I've set "tileable" to "true" when I'm loading the image, so it shouldn't be anti-aliasing. I'm reasonably sure that my maths for placing the images is correct, they're all powers of two, but even if I force the images to overlap one another by ~10px (just in case there was some background shining through) I still see the white lines:

(taken on Mac)
Any ideas?