https://www.maketecheasier.com/take-screenshot-of-login-screen-linux/ is the closest I have found to instructions to use ImageMagick to get one, but tdm seems to be different from lightdm and gdm such that I can't figure out what to change. I tried logging in and grepping XAUTHORITY from set, but it apparently cannot be found there.
I want a fullscreen screenshot that has the integrated default theme's menu expanded to show the selection of session types. Can anyone here help?