[Quote=JamesOG]I was also wondering if anyone knows how to make a color always on the bottom- with most spriting and animating there is a color that is always on the bottom- or every other color gets displayed if this color (say white) ever overlaps with other colors so that objects can overlap (like a plane flying over an island). I thought initially that the layers would suffice but since the sprite image is always a square then sprites which aren't squares have a color on the outside- like white.[/QUOTE]
That's actually a somewhat older way of doing things. I recall GameMaker used to let you pick a color for transparencies. These days though, it's best to just use transparent PNG files for your images.
Besides red, green and blue values, PNG files can have an alpha value. That not only designates that a particular pixel can be seen through, but it can be seen through to a particular degree.
If you take a look at many of the examples that come with C2, you'll see a bunch of such images. You just need an image editor that can export that format, like Photoshop or GIMP.
Just to add to this discussion too, I think optimization is an art and science itself. I'm by no means adept at it, but have the highest regard for those who know how to optimize code.