Must just be JPG artifacts then. Conversion to JPG *always* involves compression, even the larger version you just posted (where you can still see the JPG artifacting I referred to, though it's significantly lessened). Depending on what you're using to save to JPG, you may be able to get better quality at equivalent size with other settings or a different app, e.g. Photoshop's Save For Web option vs. just Save As JPG, or XnView's Export option.
With the fairly high resolution of the image and lots of fine detail, it's going to be hard to compress it to reasonable size without getting those artifacts. I'm not sure if you render at that resolution due to your screen size or some other preference, but for web sharing something more like 1280x1024 or similar is probably better for both file size/compression ratio (quality), as well as people being able to see the entire image without scrolling.
- Oshyan