Working memory improvement app needs artwork

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This customer received 7 app designs from 2 designers. They chose this app design from Martin Geyer as the winning design.
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We're building an app with exercises to improve people's working memory. We want it to be hippo themed (because hippocampus). We're looking for someone to design the artwork that we use inside the app. We're looking for cute images that we can use throughout as people play the games, usually on the smaller side. It should have a fun theme, but also one that reflects memory or exercises. Additionally, we have two exercises that need specific artwork. The first is a cross between tic-tac-toe and simon says, so we need a hippo themed image that will appear in a tic-tac-toe like board. The second is a game where one hippo will be noticeably different from the others and they will have to do something special for that hippo.
I uploaded some rough images that show some of the games using the pixar ball. We would replace the ball with these images.
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