A .png is a file format that allows you to save an image on a transparent background. Somewhat like a .gif in that respect, but it allows for a wider colour space than the .gif format, and somewhat like a .jpg, but this format is considered "lossless" file format. This file format can usually be opened by most of today's image editors - if yours can handle transparency, it will open these as simply as it opens any .jpg file; the only difference is you'll see a transparent background. This allows for easy insertion into other images.
To download the larger file, click on the thumbnail and the larger file will open in a new window (or tab). Right click on the larger file and select "save as" from the context menu and save to your hard drive. Be sure you save it as a .png file to maintain the transparency.
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