Most of theses frogs are based on the SVGs of the twitter emoji set (https://twemoji.twitter.com). Others are unique designs with other inspirations in mind.
By contributing you agree to license your contribution under the terms of the GPLv3 (for code) and CC-BY 4.0 (for graphics) licenses. The corresponding license can be found in the LICENCE and LICENSE-GRAPHICS file respectively. The following links also reference to the licenses: