This is the Planet Cute pack (original source) created by Daniel Cook, from lostgarden.com.  The author’s description for this pack::

• Building blocks, not tilesets: Instead of having complex tilesets, each block stacks nicely with pretty much any other block. If you can understand Legos, you can understand how to put together these graphics. These tiles should be useful to children, not just uber-elite game gods.
• Standardized format: All the graphics are uniform sized PNGs. The graphics will also work in almost any graphics engine out there that can do 2D sprites. Once you get your offsets right once, you never have to change them again.
• Source files included: If you do need to make changes, I’ve included the source files. If you really need to change a color, go for it. If you need a big gem, just scale up the original. Everything is a vector so you’ll alway end up with clean results.

License

This  pack is licensed under the Creative Commons Attribution 3.0 License. The author also has a license page that explains its use a little bit further.

Art attributions

If you use this pack, please include the following attribution in a visible location along with any other credits:

"Planet Cute" art by Daniel Cook (Lostgarden.com)

When possible link to blog post that discusses the original art collection. For this asset pack, it would be “Planet Cute” art by Daniel Cook (Lostgarden.com)

About the artist

Daniel Cook
I’ve been a game designer, pixel artist, toolmaker, physicist and MBA. My first job in college was on a game called Tyrian at a tiny company called Epic Megagames. These days, I’m the Chief Creative Officer at Spry Fox.