Edit to say that this is on my end (computer issue of some kind) and not a smugmug issue.
I was able to click the download button and save the original file with no problems. Can you let us know how you're going about trying to download an image? Are you viewing in a gallery, or is it on a Folder or Page filled with content blocks?
I'm also confused about your comment on uploading -- are you saying none of your photos are uploading and instead they're being filled with tiny empty images (spacer.gif)?
I've tried downloading straight from gallery and from organizer and get same results. I tried a few dozen galleries now. I haven't changed anything on the website in a while now, so I'm not sure what is causing it to happen now. I am unable to find any gallery that allows me to download jpegs.
Now I am uploading jpegs all the time, but still cannot download them.
This is extra weird now. I can download jpegs on 1 computer, but only spacer gifs on 2 others. So now I'm not sure if it is a computer problem or website problem.
When I googled this, the first thing I found was a thread from last year.
This seems to be on my end, not a smugmug issue. Sorry leftquark.
Let me know if we can help at all!