rocknbil - 4:07 pm on Jun 19, 2012 (gmt 0)
It's been too long since I worked with GIMP for precise instructions, but I think the problem is the indexed color. There's no allowance for transparent pixels.
I may have confused the issue - you can save a .png as indexed color, but as soon as you do, features not supported by .png are thrown away. That's generally why you use .gif if you're going to create indexed color.
The reason indexed color can be so lightweight is it works with a reduced palette. Let's say I have an image that will render fine with an 8 or 16 color palette. Those 8 colors are all the values that are stored in the file, and the file format maps out which pixels get which color.
Contrast that with .png, which can contain up to 16 million colors as well as transparent pixels and other data, and that's a lot of info to store.