Nothing has changed in quite a while in the image processing for the Gallery. I'm not aware of any way to add a bevel around thumbnails, probably you are talking about the background color that is used when resizing images which does create a little border that may or may not go well with the color scheme of your site. Some background color has to be used when re-sizing and we default to white, but it comes from a config setting so you can add this to your user.config, but it must be a named color with a standard named color it cannot be a hex value or any other kind of color designator.
For example you could change it to use Black like this:
<add key="DefaultResizeBackgroundColor" value="Black" />
but of course it will only affect images uploaded after the setting is changed.
Hope it helps,