There’s nothing stopping you drawing your own art and importing it into your game! You can take photographs of the landscapes around your area and use these as game backgrounds.
You could use any photo editing or drawing software (e.g. Photoshop). Photopea is a free online alternative https://www.photopea.com/ and it has a great set of community made tutorials on YouTube.
When drawing new game art, you can grab a screenshot of your current game, and then draw your new art over the top. Make a new document 960 x 540 pixels, then import your screenshot and draw over it (or import existing images). A tile is 48 x 48 pixels, and a character is roughly 100 pixels tall.
Make sure to use layers and paint onto a new transparent layer so that you aren’t painting straight onto your background. That way you can hide your background layer when you go to export your assets as PNGs.