Sets compression level (0 - 9)
Sets compression level. The level can be between 0 and 9.
Compression level 6 is the default value if nothing is set or if value outside of the bounds between 0 and 9 is set.
0 is no compression = faster but bigger files.
9 is max compression = slower but smaller.
Tests show that for most images, compression values in the range 3-6 compress nearly as well as higher levels, and do it by far faster.