Scale image quality setting is incorrect for PNG images

In ImageHelper we have method called ScaleImage. In the end of process
it does, it saves image with functions such as imagejpeg, imagepng,
imagegif, etc. These functions take quality parameter, and we pass
hardcoded quality value = 100. But imagepng function may have only 0-9
value range, and if quality is not in this range, it saves broken
image file. So resize/watermarking never worked for png images.

Note: png quality parameter values are reversed related to jpeg: 0
means no compression, and 9 means highest possible compression.

I think we need to investigate about other image saving functions,
what quality values they take.

Also, as improvement, we may add configuration value for resized image
quality, so site administrator will be able to control size-quality
balance.