Awesome Roserade, Coru! I like the smaller triangles, but I didn't have a problem with the bigger ones either (though sometimes there was white that showed between the triangle lines, like the background coming through). I don't know why you've had weird image restriction problems. Is it the forum telling you? If so, what's the message that pops up? I've posted huge images before and they were fine. Alternatively you can use the [imgsize] tag which you use like this:

[imgsize=value] img link [/imgsize]

The value depends on how big you want it. For instance, you can try 500 and see how it looks, and if it's too big, try 200, etc. I BELIEVE the value affects only the y axis, meaning that it'll be 500px tall, and the x value will scale accordingly.