Some people might not know this.
After a couple of weeks of looking at themes I realize that the theme maker will not necessarily optimize the theme for you (nothing wrong with that).
what this means is that many times the images will unecessarily be on a True color depth (16 million).
reason is that when making the images you choose colors from a 16 million pool.
Well once the image is made it is possible that it does not use more than 16 unique colors chosen out of the 16 million pool.
so if you reduce the image to 16 colors the image will look the same but the size of the image will be greatly reduce.
simple image programs like irfanview can be used to decrease color depth. some images can only be reduce to 256 colors to look the same but in most cases youll be able to reduce image size and improve performance.
Keep that in mind after you have a theme you want to keep or when making your own.

Don't use irfanview to reduce the color depth if it actually uses more colors than that. Its dithering algorithm sucks and you'll get less out of png or zip compression. (photoshop does really well if you have it, but you'll want to recompress any pngs you get out of it)

If you press 'I' irfanview'll tell you how many unique colors there are in the image. With less than 256, reducing the color depth will help a lot.

if you use bmp files you can index the image, but it's not necesary with png.

I should have said my experience on this is limmited to bmps.
Maybe someone can comment on:
Is it better to make the image the size it will actually show on the screen or when possible make the image stretched.
and if that depends on the operating system you are on.
In my case I am on w98se and make all my images the size they will show on the screen (bmps reduced to 16 colors)
I always thought about it like memory against pc calculations.
But maybe someone could make a better comment on this regard.