I've seen this trick on lots of websites, but I don't know how I could create this myself easily. It's basically text or a logo which is saved on a transparent background with a small border, to minimize the file size.
For example on Postmarkapp, they have this transparent logo, with a yellow border:
and there are a lot more of these examples on the web.
What's an easy way to create such a png file with a small border so your text or logo will seamlessly blend into the background color?