HEMI-Powered wrote:

I'm told that my Rebel writes to 2x1, I was talking about what
/I/ choose when I am post-procesing in PSP 9. Each step down the
scale makes the file size smaller and presumeably the image a
little worse. I have found that 1x2 is a very good compromise for
me. You may find that 2x1 works fine. But, as I said, if I cannot
fix the artifacts by lowering compression, I will go to 1x1, that
almost always takes care of them.

The JPEG FAQ warns that JPEG files should be saved at the same
compression settings as before, otherwise artifacts are magnified.
I'm not sure if this is also true of chroma subsampling...