hahahahaha... I just downloaded it on Windows!!!! by the way, I've been designer also so many year ago, the same situation in photoshop...
by the way, I added this line, and it shows in the screen all OK, the problem is saving it... something is wrong inside the writer handle...
Canvas.Draw(101,0,jpgLCL);
I tried it also with my mac, nothing is wrong with Gimp...
When you "Save for the Web" of course it downgrades the picture, I modified the options to 100 quality on Gimp... and I'll give you an image of the files and file sizes...