Ditched the use of modulo (%). Modulo is slow, and if you use it to just get the lower 8 bits (1 byte), you can just directly cast to byte.
Also moved one if < out of the loop for speed. The less branching the better.
Similarly replaced an if < 128 by cast to sbyte and taking its abs value.
Performance gain in writing to PNG file seems to be roughly 20% (release mode, 1000x1000 bitmap).
In ImageFramesCollectionTests I've set the culture to en-US to ensure getting english exception texts.
woutware