Okay, it seems I have found an option in Gimp that allows, after having edited a JPG, to save it using the same compression than in the original file. It should allow me in most cases to edit your JPGs without degrading the image quality when I save the file:
Concerning image quality and compression, I think there is some confusion. It is not very useful anymore, but you may read my comments if you like.
Oh and by the way, the header I make has the same quality as the original -- I didn't lower the quality at all.
If I compare the original Brik artwork and your header, I can see that the original was rotated and downscaled. It must therefore have a lower quality. Moreover, there must have been some further degradation when you saved the image as a JPG.
Finally, as far as I know none of these headers have been compressed. I at least didn't compress them -- I can't remember if you did, but I don't think so...
I only had decreased the file size of the Crysis header by saving it as a JPG. That is to say that I compressed it further. But you also saved the headers as JPGs, so that I do not know what you mean exactly.
All common image formats use some kind of compression. When you want to see or edit an image, the file is decompressed, and when you save the image, it is compressed again. The compression can be lossy (JPG) or lossless (PNG). One should be careful in the case of lossy compression, because the image quality usually decreases each time one saves the file.