Anyway 256 icons aren't much different from png 24bit icons. For our eyes there isn't difference.
I do hope you're kidding. Take a look at these two images. Download and run PowerIcons for just this test.
Alpha Blending on:

256 Color:

You may or may not see it. If you have AGA, you won't. If you have an RTG, you definately will. The alpha blending will mesh to whatever the backdrop is where as the 256 color image will just be from black to white. They should look identical on a white background with alpha blending. If you need to, go over to a friends computer who has alpha blending capabilities (even Opera or the latest Mozilla will show you). The difference is huge, and you can easily see the difference without needing someone to point it out.
Try this, turn off anti-alaising for fonts in any graphics package. The anti-alaising is a form of alpha blending. You should see a difference there as well.
:pint: