I'm accepting it for at least four months from now, probably much longer. I haven't thought about how I'll accept the material yet, perhaps I'll get an upload script up on ni2.se soon that also requires the contributors to accept an agreement. I just need to make sure it's done right from a legal point of view. There's no image size limit. If the image is too large I'll just downsample it, so it's better if the images are too large than if they're too small. I'm after objects, not patterns right now. The main important thing is that they blend nicely to the background, and that the edges are anti-aliased (hence the need for an alpha channel).