Icon improvements: converted from ICO to PNG, optimized PNGs

Note that ICO files are unsuitable as package icons, because they have several drawbacks:
* Unlike PNG, ICO files are a custom format by Microsoft, not an official W3C standard.
*  Less application support.
* Many ICO files store multiple images. This is completely unnecessary, because browsers show only the first image of the ICO file. The others only increase the file size with no use.
* Resolution problems: Even if you have a 16 × 16 px and a 128 × 128 px image in the ICO file, browsers don’t automatically pick the highest resolution, but the first, which is 16 × 16 px in this case. That’s bad.

So remember to not use *.ico files for your packages.