The current profile's avatar looks like this:
Zoom in and you can clearly see what's wrong with this image:
I work at an advertising agency, and I always have to remind people to use .png or .gif files instead of .jpeg for logos and typographic images. This is because JPEG compression always results in visible artifacts. In a photographic image these are not that apparent. In a logo however, they tend to be distracting.
If that's not possible my advise is to use .jpg with little or no compression (high quality) to prevent the most visible artifacts like you see above (the dirty white around and between the lines of the logo).
Examples (I couldn't find a high-res version)
.jpg (quality: 100, some artifacts, but not distracting at smaller sizes)