After I upload an asset (png), the image loses quality. I changed the defaultImageQuality from its default value of 82 to 100 but didn't make a difference. I can't add these assets via FTP as I need users to be able to add them from the admin panel. Any advises?
Converting my comment(s) into an answer...
Make sure you are using the ImageMagick library to handle image manipulation (instead of the default GD library). It provides much better results.
Craft will "clean" every image that gets uploaded. This prevents anyone from accidentally (or intentionally) uploading malicious scripts to your site hidden within an image file.