(Debian 10, EXWM, Emacs 27.1)
Relative banner paths don't work for me. Absolute paths do (text and image).
When evaluating the dashboard-banners-directory function, I get two slashes "//" between my dashboard library directory and banner directory.
The dashboard-banners-directory function uses the file-name-directory function, and the docs say:

On GNU and other POSIX-like systems, a string returned by this function always ends in a slash. On MS-DOS it can also end in a colon. 

So, I assume that's why it's evaluating to a path with an extra slash, and why relative banners don't work for me.

(defconst dashboard-banners-directory
  (concat (file-name-directory (locate-library "dashboard")) "/banners/")
  "Default banner directory.")

I've only been using emacs for like a week and don't know enough to get this repo to build and see if changing "/banners/" to "banners/" would fix relative banner paths for my system.