edit: hmm, as I was writing this reply the extra border has disappeared. Anyways, as I was saying...
This is a result of some changes I made in HEAD which I think Urs has committed to stable (at least that is how I assume they turned up on Moodle.org). It seems as if only part of the changes have been transferred across e.g. I uploaded new default images without borders at all which aren't in evidence and there appears to be no CSS applied to the default user images (hence the difference in borders). In fact, looking at the HTML code the CSS classes seems to be different from my code too, though I'm not sure why at the moment.
The basic idea is that having a border embedded into the image isn't as flexible as drawing it with CSS instead. The problem in transition of course is that some images already have borders embedded within them (at upload time) so drawing a second border with CSS is overkill.
This obviously isn't a problem for new installations, but there are also several things that could be done to ease the transition for established sites where many images have already been uploaded and had a border added.
ask people to re-upload their user images (the HEAD image upload script no longer adds borders)
run a script that replaces the current black borders with a mid-light grey border or resizes the images so that they are 2 pixels larger in height and width (though both these scripts will cause a generation loss as it involves creating a new jpeg and resizing might look odd, probably needs practical testing to see which looks best)
replace the black border in the CSS with a mid-grey. Older images will still have a thicker border, but it won't be as thick as 2px of black and newer images will have a thinner border. (Which I happen to think is better as the thinner black border is, in my opinion, quite visually brutal, even when only 1px thick.)
