bootron:It's true that we now save your original image, but still it is necessary for us to compress the image so that we can save bandwidth. So to set the record straight, all images are resized and compressed when displayed on the website in the various places. But if you click through any image, you can ultimately see the original if you so desire.
This more or less answers my question. I got a lot of responses in the thread about how to resize images (which I've already been doing, thanks). My current idea of what's happening is the JPEG quality slider is being moved from 100% to something more like 50% thus bringing out the horrible JPEG artifacts.
My next tactic will be to try something like dropping the JPEG quality to 50% and trying to make the image look good there before uploading it.