Power Hearts Background
To quote Wikimedia Commons where they have this image as .gif, "This image only consists of simple geometric shapes or text. It does not meet the threshold of originality needed for copyright protection, and is therefore in the public domain. Although it is free of copyright restrictions, this image may still be subject to other restrictions." The copyright restrictions in this case is most likely an issue if you use it in a cartoon because The Power Puff Girls already use this. Otherwise, feel free to use this for personal reasons, and as an artist, I don't care if you use it for commercial reasons, just make sure you aren't using it in a way that will attract The Power Puff Girls' legal representatives. No attribution required.
These hearts were done in Photoshop. They were not copied from anything other than Photoshop's shapes.
Courtesy of Michelle Grewe
Find me on crumpetsandbollocks.com and michellegrewe.com