Put an background image on the right-bottom (or anywhere else) of Typora.

typora-background-image.css

/*
  Add this to ~/.config/Typora/themes/github.css (or the name of your theme        
  The background image should be put in the theme directory
*/

content::after {
    content: '';
    background-image: url(./github/image.png);  /* relate path is required */
    height: 250px;                              /* set by yourself */
    width: 200px;                               /* set by yourself */
    position: fixed;
    right: 7px;                                 /* avoid overlapping the slider. */
    bottom: 30px;                               /* avoid overlapping the bottom status bar. Use "0px" for macOS. */
    background-repeat: no-repeat;
    background-size: contain;
    opacity: 0.3;
    pointer-events: none;
}