I use paint. Go to resize and change it to 20% thats the size I use and havnt had any problems. It depends on the original size though. Mine starts out at 8MP
I use Flickr to post pics all the time. You have to click the share drop down above the selected picture. Then copy the .jpg file portion that drops down. Then open the link image button and paste the file and...
I've been using photobucket, and I just resize after I upload to photobucket.
I just go to edit, then resize, then, type in 800 for the width, then save it, done.
There are probably better ways, but this seems to work.