[c#] Converting a byte array to PNG/JPG

I am currently working on an application that requires high-performance conversion of an unpadded byte array to either a PNG or JPEG. The image format doesn't matter, just as long as it's fast.

I have tried the .NET libraries and the performance is very bad. Can anyone recommend a good freeware library for this?

EDIT: the byte[] is an 8bit grayscale bitmap

I like Imagemagick. http://www.imagemagick.org/script/api.php

There are two problems with this question:

Assuming you have a gray scale bitmap, you have two factors to consider:

1. For JPGS... what loss of quality is tolerable?
2. For pngs... what level of compression is tolerable? (Although for most things I've seen, you don't have that much of a choice, so this choice might be negligible.) For anybody thinking this question doesn't make sense: yes, you can change the amount of compression/number of passes attempted to compress; check out either Ifranview or some of it's plugins.

Answer those questions, and then you might be able to find your original answer.