You can upload custom textures for individual worlds for 3 coins. This will change the default appearance on the map and the world's page for everyone to maybe enjoy.

The link to upload a texture to a planet is on the planet summary screen under it's current texture.

Please make sure you use a seamless texture that passes as a planet's surface. If an uploaded image fails the technical requirements, it will be rejected and you will be reimbursed. If it is an obvious shenanigan, it will be rejected without reimbursement. If you waste someone's time for three dollars, be prepared to lose said dollars.