This problem is even worse when an image has been flattened onto a solid background. Even once I knew how to create, I needed the source to be able modify others' work. Fortunately, this problem is potentially easy to solve. On my PII 366, this opperation takes about 6 seconds on a 6 layer 192x192 source image. Icons are not the only type of image that is used in software.
Original photographs are, by their very nature flat images in terms of layers.
It would not make sense to expect a more source-like file than just a flat image file. However they are often stored as .jpg files which are lossey. The third type of image I would like to discuss is computer-created images such as desktop backgrounds. Propaganda project created tiling desktop backgrounds. Despite the GPL liscence on the images they were only released in .jpg format.