SVG (Scalable Vector Graphics) is an open standard used to describe two-dimensional vector graphics. It is great for diagrams and shapes, while PNGs are better for photographs. SVG files are a bit like instructions on how to make an image. This means that they can be resized without looking pixelated or blocky.
For more information, visit: https://www.w3.org/Graphics/SVG/
An excellent tool with which to create SVG images for use in Tux Paint is Inkscape, a high-quality Open Source interactive drawing program available for Windows, macOS, and Linux. There are also other applications that can produce SVGs, both free/open source, and commerical/proprietary.