NOTE: Inkscape has been updated since the publishing of this video. It may have a different look, but the general steps will remain the same. Certain sliders or options (within 'Trace bitmap') may also work better/worse.
Use Inkscape to convert images into a 'svg'. Then use Tinkercad to import that file, and create your own coaster. Or turn that coaster into a magnet by simply gluing a magnet to the back.
0:00 - Introduction
0:44 - Installing Inkscape
1:30 - Examples and choosing a good image
3:00 - Creating a SVG with Inkscape
8:14 - Tinkercad
21:24 - Cleaning up the imported SVG
23:44 - Coaster vs magnet
25:01 - Try painting them!